Weighed in the Balance

reminder of the antiquity of the de- vice. The illustration appeared as frontispiece to an early catalog issued by L.Oertling of London; this firm has...
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Weighed in the Balance John T. Stock Department of Chemistry University of Connecticut Storrs, Conn. 06268

The history of balances is really The modern precision balance has an ancestry that seems to go back be­ yond recorded history. Figure 1 is a reminder of the antiquity of the de­ vice. The illustration appeared as frontispiece to an early catalog issued by L. Oertling of London; this firm has specialized in the making of pre­ cision balances since the 1840's. Al­ though some single-pan devices such as the sliding-weight Roman steel­ yard and the sliding-center pivot bismar also have ancient origins (2), precision balances were almost en­ tirely of the two-pan variety until comparatively recently. In fact, the term "balance" implies the use of two pans. Sensitivity and accuracy are prime requirements for any measuring in­ strument. Convenience, including speed of operation, can come later. With any normal form of balance, the "accuracy" obviously depends upon the goodness of the available set of weights. However, the balance will be of little use unless it is sensitive and precise. Ideally, the bearings or pivots of a balance should be frictionless, and the beam and its appendages should be infinitely light. However, precision, including constancy of sen­ sitivity under varying load, implies the use of an extremely rigid, and thus quite possibly massive, form of beam. The history of the balance is, in fact, very largely the history of its bearings and its beam. A very early balance consisted of little more than a rod or bar, which was suspended by attaching a cord to the middle. The pans, or merely the masses to be compared, hung from cords that were affixed to the respec­ tive ends of the beam. Flexure of the cords provided the "bearings" of this

the history of the development of the bearings and the beam of the two-pan balance to ensure both sensitivity and precision crude device. Although the rod-like beam persisted, more sophisticated bearings began to develop. Knife-edge bearings were certainly in use quite early in the 16th century (2). The swan-neck beam, which can be traced back to about the same period, is still encountered in small commercial scales for the weighing of coinage and the like. Figure 2 shows a swan-neck beam balance that is reputed to have an association with Joseph Black (1728-1799). The swan-neck design allows the half-beam lengths to be equalized merely by bending the ex­ tremities toward or away from the center of the beam. Henry Kater (1777-1835), army of­ ficer and scientist, turned the rather crude swan-neck balance into the semiprecision device shown in Figure 3, which is reproduced from the en­ graving of 1821 (3, 4). The bending of the swan necks is nicely controlled by screws α and b, which are arranged at right angles. This permits both equal­ ization of the half-beams and the lining-up of the three bearings. Accord­ ing to Kater, this balance, "manufac­ tured by Mr. Robinson, of Devon­ shire-street," was provided with weights "from 1/100 of a grain up to 100 grains." This implies that mass differences of less than 1 mg could be detected. No example of the KaterRobinson balance has been found, de­ spite searches spread over the past seven years.

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Kater (4) also refers to an addition that "Dr. Wollaston has contrived." W.-H. Wollaston (1766-1828) was the discoverer of palladium and rhodium. In the slightly modified form shown in Figure 4, this additional device was used in the later balances made by Robinson and his successors. Two pairs of jointed strips are attached to a short strip that is screwed to the baseboard of the balance, so that the narrowed handles project from elon­ gated slots in front of the case. The longer portions of the strips each carry two pins that point upward. After the beam has been arrested, the handles are manipulated until the pins make lateral contact with the re­ spective pans. Any out-of-balance be­ comes obvious when the handles are returned to the position shown after the arrestment has been released. The balance shown in Figure 5 was acquired by the Royal Scottish Muse­ um, Edinburgh, in 1928. Unfortu­ nately, there is neither maker's mark nor date. This beautifully constructed instrument has a triangulated opentype 11-in. beam with a steel knife and a horizontal upper rail divided and numbered for use of a rider. The rider, long pointer, and arrestment control by rotation of a knob are fea­ tures that can still be found in pres­ ent-day two-pan instruments. Partic­ ularly interesting is the adjusting sys­ tem for the swan necks. Both ends of the beam have a pair of screws ar­ ranged at right angles, as shown in Figure 6. With this system, a setting can be found that combines liningup, equalization, and the fixing of the pointer at right angles to the plane containing the knife and end bear­ ings. Simplicity is a virtue of the swan-

Figure 1 . "3000 years ago the Egyptians weighed the souls of the dead in the HALL OF PERFECT JUSTICE" Figure 2. Large balance supposed to have been used by Joseph Black Figure 3. Kater-Robinson balance

Figure 4. Diagrammatic plan view of Wollaston pan arrestment Figure 5. Balance with fully independent swan-neck adjustments FIGURE 1.

FIGURE 4

Figure 6. C l o s e - u p of a d j u s t a b l e s w a n n e c k b e a m end

neck end bearing; the hook of the pan support is merely inserted into a sharpened hole at the end of the beam. Ease of assembly is important in portable balances, because safe transport is possible only after the mechanism has been taken apart or otherwise secured. Probably the most refined of all swan-neck balances is the Portable Assay Balance, once made by Oertling. A fine specimen of this unique instrument is in the London Science Museum. This balance has a capacity of 2 grams and is sensitive to 0.1 mg or better. When closed for transportation, the instrument fits into a small leather case with a shoulder strap and can be easily carried while prospecting. The all-pervading influence of the tax collector is nothing new. Toward the end of the 18th century, the British Government sought the advice of Sir Joseph Banks, then President of the Royal Society, concerning the assessment of excise duty on potable spirits. This appeal resulted in the need for the accurate determination of the specific gravities of alcoholwater mixtures. A superb balance that is now in the Science Museum was used to carry out this work (5). This balance was made by Jesse Ramsden (1735-1800), a famous London instrument maker (Figure 7). Ramsden was an inventive genius and a perfectionist. Completed instruments that did not meet his rigid standards were invariably rejected. Ramsden introduced the form of beam that consists of two hollow cones joined at their bases. The Ramsden design was adopted by many other makers. Apart from Ramsden's own version, several balances of this type are still in existence. Figure 8 shows an example by

F i g u r e 7. J e s s e R a m s d e n

F i g u r e 8. C o n e - b e a m b a l a n c e by T h o m a s J o n e s

976 A · ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973

Figure 9. Diagrammatic representation of end bearings (a) Knife-and-ring, side view and end view; in practice, small side plates are provided to restrict lengthwise movement of knife, (b) Side view of roof-type bearing

Thomas Jones (1775-1852), whose trade label proudly claims him to be "Pupil of the late Mr. Ramsden." The double-cone beam of this balance has a central steel knife that turns on circular agate planes. As is usual in this type of balance, end bearings are of the loop- or knife-and-ring form (Figure 9a). Such bearings are selflocating, so that the balance needs only a simple form of arrestment. This merely has to take care of the separation of the center knife from its plane and the proper relocation of this knife on this plane when the arrestment is released. The sensitivity of the balance is adjusted by rotation of the small knob above the middle of the beam. This causes an internal weight to rise or fall. Because the forces that act upon the beam of a balance are confined to the vertical plane, the great horizontal rigidity provided by the doublecone beam is unnecessary. A flat beam that has considerably greater height than thickness is obviously more practical. Furthermore, a light but rigid flat beam can be made by combining tapering with judicious removal of metal. Although Ramsden's prestige caused many of his successors to adopt the cone-type beam, balances with quite well-designed flat-type beams were certainly available during Ramsden's lifetime. An example is the instrument signed "Haas & Hurter London," in the Teylers Museum, Haarlem, Netherlands. These makers dissolved their partnership in 1795 (6). Both members of a knife-and-plane bearing have simple geometric forms. This enables either or both members to be made of hard, corrosion-resistant, but intractable materials such as agate, quartz, or gemstones. Because these simple forms can be finished to a high degree of perfection, the loading of the bearing can be distributed over a quite long line of con-

tact. The loading of a knife-and-ring bearing is concentrated into two quite small regions. It is therefore highly desirable to use knife-and-plane bearings at the ends of the beam, as well as at the center. To minimize wear, the bearings should be unloaded except when actual weighing is in progress. However, a knife-and-plane bearing is not self-locating, so that release of the arrestment mechanism must cause each knife to take up a fixed position upon its plane. The arrestment system for a beam that employs this type of bearing throughout must therefore be more complex than the arrangement that suffices when ring-type end bearings are used. Despite the fame of the balance shown in Figure 10, details concerning its conception, date of manufacture, and certain aspects of its subsequent history are still lacking. Most of our information rests upon the following memorandum, which was prepared by John George Children, whose excellent home laboratory was sometimes used by his friend Sir Humphry Davy. "This balance (of rude exterior, but singular perfection) was made by Harrison according to the plan and by the order of Henry Cavendish Esq. and passed at his death to his cousin and heir, Lord George Cavendish. By him it was presented to Sir Humphry Davy, together with the greater part of Mr. Cavendish's philosophical apparatus. Sir Humphry Davy gave it to J. G. Children who now presents it in token of his sincere regard, and in acknowledgment of innumerable friendly offices to Alexander Garden. Jan 1st, 1830." Apart from its association with Henry Cavendish (1731-1810), and therefore with early quantitative chemistry, the balance is important because of its advanced design. Knife-and-plane bearings, used as the beam ends as well as at the center,

Figure 10. Cavendish-Harrison balance

are controlled by a rather complicated but effective arrestment system ( 7). If this is the balance used by Cavendish in his study of the composition of water, the date of manufacture cannot be much later than 1780. Children's memorandum is usually taken to mean that Cavendish was the designer and that the maker was the great horologist John Harrison (1693-1776) of marine chronometer fame. An instrument maker of this caliber may well have suggested some of the design features. Although John Harrison may have made the balance in his more active years, the actual constructor is more likely to have been his only son, William. William

ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973 · 977 A

Harrison (1727-1815) turned to other pursuits in later life, but he was an instrument maker of no mean order and inherited his father's equipment and "drawing books." William Harrison is named as a legatee in Cavendish's will. When Davy left the Royal Institution in 1813, he probably transferred the balance to Children's laboratory. That Children should have given the balance to Alexander Garden, instead of returning it to the Royal Institution, is a real puzzle. I am indebted to A. K. Henriksen for the following suggestion. According to the obituary on J. G. Children (8), " . . . he succeeded in discovering a process by which silver might be obtained (from its ores) without the use of mercury . . . and at less cost. The right of using this process was purchased by several of the companies and a considerable sum was the fruit of it." Alexander Garden is described in the London Directory of 1823-4 as a "manufacturing and experimental chemist." It is possible that his "innumerable friendly offices" were connected with the process by which Children made his fortune. The minutes of the May 18, 1868, meeting of the Managers of the Royal Institution include a resolution to accept "Mr. Cavendish's balance" as a gift from Felix R. Garden. The donor, Alexander Garden's son, inherited his father's business, although he eventually disposed of it. So finally the balance returned to the Royal Institution and is presently in its newly opened Faraday Museum. Another form of self-locating end bearing makes use of a knife and a "roof", i.e., a block that carries a Vshaped recess in the underside (Figure 9b). End bearings of this type are used in the balance shown in Figure 11. This instrument is in the Manchester Museum of Science and Technology and is signed "Liebricht Giessen." There is some evidence that the balance once belonged to the Manchester scientist, Edward Schunk, who studied with Liebig around 1840. Writing toward the end of the last century, Dittmar (9) points out that plane bearings "introduced by Robinson of London many years ago" were undoubtedly superior to roof-type bearings. Dittmar laments, ". . . yet, after Robinson's death, Oertling was almost the only balance maker who followed him in this respect." This "Robinson" and the "Mr.

Figure 1 1 . B a l a n c e with r o o f - t y p e end b e a r i n g s

Figure 1 2 . Center b e a r i n g of R o b i n s o n - t y p e b a l a n c e

Figure 13. Robinson & B a r r o w b a l a n c e

978 A · ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973

Figure 14. B a l a n c e used by J o u l e

Robinson" referred to by Kater (4) was Thomas Charles Robinson (1792-1841). The existence of the Cavendish-Harrison balance shows that Robinson did not actually introduce the knife-and-plane end bearing. However, he produced a really practical design with a compact arrestment system (10). His trade announcements list balances with beam lengths of 5%, 8, and 10y2 in. He adjusted these to "turn with one thousandth of a grain" (i.e., 0.065 mg) and offered "crystal" knives as alternatives to those of steel. Portability is a feature of these balances. The center plane is cemented to a brass plate, which is held on the top of the open-type column by two large thumbscrews (Figure 12). When these are removed, the plate can be slipped out, so that the beam can be lifted off and packed in a drawer in the base of the balance. This method of storage dictates the use of a short pointer and of flexible cords or chains to carry the pans. Child (11) considers Robinson to have been the first to manufacture balances on a commercial scale and states that the first precision balances used in America were Robinson ones. The present author wishes that he could find one of such balances (or even some evidence of their use) in this country!

Robinson's apprentice, John Dover (1824-1881), was awarded a medal for a 10y2-in. Robinson-type balance that he showed in the international exhibition that was held in London in 1851. However, his later activities appear to have concentrated upon the making of surveying instruments. Soon after Robinson's death, his business was acquired by Henry Barrow (1790-1870) and carried on under the name of "Robinson & Barrow" (12). Although he was a fine instrument maker, Barrow was content to retain the original Robinson design. Figure 13 shows a fine example of a Robinson & Barrow balance that is the property of the British Museum (Natural History). This balance was still in use when first seen by the author in 1970. It has platinum pans and chains and is associated with M. H. Nevil Story Maskelyne. He was the first Keeper of Minerals in the British Museum over the period 1857 through 1880. In view of the success of Robinson's design, it is not surprising to find Robinson-type balances that carry the names of other makers. One such balance, signed "Adie & Son EDINBURGH," is in the Royal Scottish Museum (13). Although he accepted the fundamental design, Adie did alter some details of construction. However, the alterations were no

more than to be expected when a master craftsman utilizes the design of one of his peers. Adie had already made at least one other precision balance and was acquainted with the technology of weighing devices. I am indebted to K. R. Farrar of the Department of History of Science and Technology in the University of Manchester for access to the Robinson-type balance shown in Figure 14. This balance, signed "Dancer Manchester," is associated with James Prescott Joule (1818-1889), best known for his determination of the mechanical equivalent of heat. Joule worked with Lyon Playfair on atomic volumes and densities (14). L. L. Ardern, Deputy Librarian of the University of Strathclyde, has provided me with photocopies of two of Joule's letters to Playfair. On February 2, 1846, Joule wrote, " . . . Dancer . . . says I shall have my balance by the end of this week." Then, on February 9: "I am now working with my new balance which is really an excellent instrument. . ." This balance has suffered considerably since Joule's time, and the case is now missing. Preliminary inspection showed a surprising resemblance to existing Robinson or Robinson & Barrow balances. Through the courtesy of D. S. H. Cardwell, Honorary Curator of the Joule Collection, and of A. A. Moss, Keeper of Mineralogy in the British Museum, the Joule barance was brought from Manchester to London and compared side-by-side with the Robinson & Barrow balance shown in Figure 13. Not only are the balances identical in design, but also in practically every dimension. Apart from the pans and their suspensions, the most obvious difference is in beam length (i.e., the distance between end knives). This length is IOV2 in. in the Joule balance and 10%6 in. in the London balance. The Robinson adjustment system allows the beam length to be slightly altered, so that this difference is not significant. In any case, another Robinson & Barrow balance that is in the Whipple Science Museum, Cambridge University, has a beam length that is slightly greater than 10% in. John Benjamin Dancer (1812-1887) is most widely known as the father of microphotography (15, 16). However, his chief activities in Manchester concerned the manufacture and sale of optical and scientific instruments. His trade advertisements and his autobiography (16) stress that he was

ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973 · 979 A

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a n inventor as well as a n i n s t r u m e n t m a k e r . It t h u s seems s t r a n g e t h a t he should slavishly copy an i n s t r u m e n t t h a t was designed by someone else a n d , furthermore, was still in p r o d u c ­ tion. O n e would have expected a t least a few minor i n n o v a t i o n s . A careful e x a m i n a t i o n of M a n c h e s ­ ter newspapers a n d directories for t h e a p p r o x i m a t e period 1840-1850 h a s been carried out by J a n e Walker. T h i s search has failed to reveal any evidence t h a t D a n c e r m a d e b a l a n c e s or even offered t o do so. Although Dancer may have constructed the Joule i n s t r u m e n t , this would have in­ volved t h e m a k i n g of tools a n d jigs— a n d t h e acquisition of b a l a n c e " k n o w how"—all for a "one-off" i t e m ! A m u c h more obvious a n d economic al­ t e r n a t i v e would be to acquire a b a l ­ ance from Barrow a n d to apply t h e D a n c e r n a m e . Joule, t h e customer, would not suffer; he would get t h e finest b a l a n c e available anywhere! Robinson was m a n u f a c t u r i n g preci­ sion b a l a n c e s with a b e a m length of only 5% in. before 1830. However, t h e view t h a t high sensitivity was best achieved by use of a long b e a m pre­ vailed for a long t i m e . T h e n , in 1866, P a u l B u n g e (1839-1888) showed t h a t a highly sensitive b u t rapidly swing­ ing s h o r t - b e a m b a l a n c e was b o t h t h e ­ oretically possible a n d q u i t e p r a c t i ­ cal. T h e first s h o r t - b e a m b a l a n c e with a n a l u m i n u m b e a m was m a d e by Florenz Sartorius (1846-1925) who in 1870 founded t h e business t h a t still bears his n a m e . A b o u t this t i m e , t h e precision analytical b a l a n c e acquired t h e general a p p e a r a n c e t h a t r e m a i n e d essentially u n c h a n g e d u n t i l t h e popu­ larization of t h e " s i n g l e - p a n " b a l a n c e (17). T h e balances used by m a n y fa­ mous scientists have been a d m i r a b l y surveyed by Oesper (18). T h e m a i n stem of b a l a n c e develop­ m e n t h a s , of course, p r o d u c e d b r a n c h e s . T h e s e have led to devices such as m i c r o b a l a n c e s of n o r m a l , tor­ sion, or electromagnetic form a n d to recording balances (19). An inter­ esting survey of m o d e r n laboratory balances has been m a d e by H i r s c h (20). References

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(1) F. G. Skinner, "Weights and Mea­ sures," ρ 73, Η. Μ. Stationery Office, London, England, 1967 (Science Muse­ um paperback; available from British Information Services, 845 Third Avenue, New York, N.Y. 10022). (2) L. Sanders, Trans., Newcomen Soc, 24, 81, 1943-45 (published 1949). (3) Anon., Quart. J. Sci., 11, 280 (1821). (4) H. Kater, ibid., 12, 40 (1822). (5) J. T. Stock, "Development of the Chemical Balance," ρ 13, Η. Μ. Stationery Office, London, England, 1969 (Science Museum paperback; available from British Information Ser­ vices, 845 Third Avenue, New York, N.Y. 10022). (6) G. L'E. Turner, private communica­ tion, June 7, 1968.

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(7) J. T. Stock, "Development of the Chemical Balance," ρ 11, Η. Μ. Stationery Office, London, England, 1969. (8) Gentleman's Mag., 37, (N.S.), 622 (1852). (9) W. Dittmar, in "Thorpe's Dictionary of Applied Chemistry," 4th éd., Vol 1, ρ 587, Longmans Green & Co., London, England, 1937 (This is essentially the account that Dittmar wrote for the first edition, which appeared in 1890). (10) J. T. Stock, J. Chem. Educ, 45, 254 (1968). (11) E. Child, "The Tools of the Chem­ ist," ρ 81, Reinhold, New York, N.Y., 1940. (12) J. T. Stock, Chem. Brit., 7, 385 (1971). (13) J. T. Stock and D. J. Bryden, Technol. Culture, 13,44(1972). (14) J. P. Joule and L. Playfair, J. Chem. Soc, 1, 121 (1848). (15) H. Garnett, Mem. Proc. Manchester Lit. Phil. Soc, 73, 7 (1928-9). (16) W. Browning, ibid., 107, 113 (1964-5). (17) J. T. Stock, "Development of the Chemical Balance," ρ 30, Η. Μ. Stationery Office, London, England, 1969. (18) R. E. Oesper, J. Chem. Educ, 17, 312 (1940). (19) J. T. Stock, "Development of the Chemical Balance," pp 36, 44, H. M. Stationery Office, London, England, 1969. (20) R. F. Hirsch, J. Chem. Educ, 44, A1023 (1967); 45, A7 (1968). Work partially supported by the University of Connecticut Research Foundation.

J o h n T. Stock is professor of chemis­ try a t t h e University of C o n n e c t i c u t a n d received the P h D a n d D S c de­ grees from t h e University of L o n d o n . After extensive industrial a n d aca­ d e m i c experience, he joined t h e fac­ ulty of t h e University of C o n n e c t i c u t in 1956. His principal interest is ana­ lytical c h e m i s t r y in its widest sense. H e is t h e a u t h o r of t h e s t a n d a r d ref­ erence work, " A m p e r o m e t r i c T i t r a ­ t i o n s , " a n d writes the biennial re­ views on this topic for A N A L Y T I C A L C H E M I S T R Y . T h e d e v e l o p m e n t of a p p a r a t u s a n d t e c h n i q u e s for t h e t e a c h i n g of c h e m i s t r y a n d t h e history of scientific i n s t r u m e n t s has con­ cerned h i m for m a n y years. R e m e m ­ bering boyhood fascination, Dr. Stock m a d e a detailed e x a m i n a t i o n of t h e magnificent collection of balances in t h e L o n d o n Science M u s e u m d u r i n g his 1965 s a b b a t i c a l leave. H e is pres­ ently s t u d y i n g historic electrical m e a ­ suring i n s t r u m e n t s as well as bal­ ances. His A C S c o m m i t t e e m e m b e r ­ ships are on E x a m i n a t i o n s a n d on Awards.