Eben Horsford (1818-1893) and the Measurement of Electrolytic Resistance John T. Stock University of Connecticut. Storrs, CT 06268 Eben Norton Horsford, a major figure in the development of American agricultural science, bad a wide range of interests. His apparently single incursion into the field of fundamental electrochemistry is not widely known. Horsford was born on July 27,1818, in Moscow (now Leicester), New York, the son of a progressive farmer. He soon developed a strong interest in geology and, while still in his teens, began such diverse activities as the making of surveys and teaching in school. He graduated as a civil engineer in 1838 and, two years later, he began to teach mathematics and natural history a t the Albany Female Academy. Around 1842, he became interested in Liebig's views by reading this scientist's hook Animal Chemistry and Organic Chemistry. He also made the acquaintance of John W. Wehster, the Harvard geologist and chemist, who had edited the American versions of Liehig's works. Anxious to better himself, Horsford applied for a professorship a t the University of Pennsylvania in 1844. He was unsuccessful. This disappointment, coupled with Webster's advice, probably led Horsford to the conclusion that the learning of agricultural chemistry in Liehig's laboratory would he valuable. He arrived in Giessen in November, 1844, and remained there for two full years. After taking a large number of courses during the first year, he began to workunder Liehig's direction. A study of the nutritive value of animal feeds involved the performance of numerous routine nitrogen determinations, and his imaginative mind needed relief from these tedious operations. Accordingly, he sidetracked into various other areas. One of these was an early precursor of electrolytic theory. This probably led him to attempt to measure the resistances of solutions of electrolytes. Although Georg Ohm bad published his work on the resistance of metallic conductors in 1826, there were no standards of resistance, and very little had been done concerning the resistance of solutions. In such measurements, interfering effects occur in the region of the electrodes that provide electrical continuity between solution and measuring equipment. These effects, loosely described as due to electrode polarization, augment the true resistance in an often irreproducible fashion. Nowadays, these effects are eliminated or minimized by alternating-current methods, either with a normal pair of electrodes or by the coupled-coil electrodeless system. The latter system, used industrially, can handle highly conducting solutions as well as poor conductors. Another approach uses constant direct current in conjunction with a four-electrode system (1, 2). The actual measurement is of the potential drop across a pair of identical electrodes that define a fixed length of a uniform column of solution, through which the known current is passing. Horsford became aware of polarization effects in his studies on the measurement of the resistances of various solutions of electrolytes. His 1847 paper explains that he had to return to America before his investigations were complete, but he was publishing the essentials because they might serve some practical need (3). His "cell" was a well-varnished rectangular wooden 700
Journal of Chemical Education
Figwe 1. Horsford's "trough" cell.
trough of dimensions 30 X 7.5 X 7.5 em. Two parallel wooden partitions, the inner faces of which were entirely covered by platinum plate electrodes, were inserted, as shown in Figure 1. One partition could be moved, so that the distance between the electrodes could be adjusted. By filling the trough to various depths, a liquid column of known length and cross section could be defined. Horsford set the interelectrode distance a t 2.5 em, then connected the cell in series with a battery, a tangent galvanometer. and a variable resistor bavina German silver wire as the resistor element. The resistor was adjusred to a value RI (anually m a known length of resistor wire in the circuit!. to give a convenient galvanometer deflection. Then the interelectrodedislance was increased to d c m (usually 5.0 cmi. and the original galvanometer deflection was regained by decreasing the resistor value to Rz. Then (RI - Rd represents the resistance of a column of solution of length ( d 2.5) cm, and of cross section defined by the width of the trough and the depth of the solution. Using sulfuric acid solution of specific gravity 1.10 (i.e., approximately 1.7 M) and a solution depth of 2.75 cm, Horsford attempted to show that the results were independent of the current strength indicated by the deflection of his galvanometer. His results, each the average of at least three determinations, are listed in the table. As in all of his data, the resistance is expressed in terms of the resistor wire. Two factors must he considered in viewing these results. One is the comparatively crude Eflecl of Current Strength on the Measured Resistance of Dllute Sulfuric Acld Solutlon Gelvanometer deflection, (') Solution resistancea
10 20.67
20 21.25
30 20.00
Figure 2. Relationship beween r w i a a n c e d dilute sulfuric acid and column length.
nature of Horsford's apparatus. The other is the sensitivity to temperature of the resistance of electrolyte solutions. Horsford states that his experiments were done a t 18-20 "C. Varying the current would certainly change its heating effect and hence the secondary effect of convective stirring. Although unlikely to he large in experiments of this kind, change in concentration due to electrolysis is another possible source of error. Usine the same denth of the same sulfuric acid solution. ~oisfo;d aimed toshbw that the resistance was proportional to the lenethof the columnofsolution. I.'irmre 2. constructed from ~o&ford's data, indicates that h s supposition was correct. He measured the resistances of a few sulfuric acid solutions of higher concentrations and made limited observations on solutions of salts such as sodium chloride, potassium chloride, and magnesium chloride. Similar observations were made with copper electrodes in copper sulfate solution and with zinc electrodes in zinc sulfate solution. A footnote indicates that he had trouble with the latter system and that be regarded the zinc sulfate data as being approximate only. Horsford was obviously unaware that the British physicist Charles Wheatstone had already made use of the same princiole that Horsford described. In his Bakerian Lecture of 1843 to the RuyalSociety, Wheatstoneourlined this'khange of electrode distance" method and illustrated his alass cell (4). Wheatstone stated that his measurements on ionducting liquids were not sufficiently numerous to enable any general conclusions t o he drawn, that he was still experimenting, and that he was deferring an account of these experiments to a future occasion. Typical of the use of Horsford's method by others is the 1859 study by Schmidt of the resistances of solutions of sodium chloride and of potassium nitrate (5).Schmidt made his measurements over the approximate temperature range
1 7 3 0 OC, quoting individual temperatures to the nearest 0.1 "C. This enabled him to list the temperature coefficients of resistance of his solutions. In a monograph published in 1861 (6),Wiedemann surveyed the attempts made by earlier workers to measure the resistances of electrolyte solutions and, as far as possible, compared the various results. One of his conclusions is that the results obtained by Horsford's method are to be preferred because the method eliminates the effects of polarization. The classic text of Kohlrausch and Holborn appeared in 1898 (7).By then, the application of alternating current to the examination of electrolyte solutions was well established, having been introduced by Kohlrausch and Nippoldt in 1868 (8). This text briefly surveys the earlier, directcurrent, techniques and the attempts to eliminate polarization effects. Horsford's 1847 paper (3) is the earliest one mentioned (7). With John Webster's support, Horsford became Rumford Professor a t Harvard University in 1847. He was determined to build a fine teaching laboratory and, despite funding ~rohlems,had succeeded hv the end of 1848. In those davs. a professor's salary depended on the number of his students. The enrollment in Horsfurd's courses was never hinh enouah to give him a comfortable income. H e struggled tdlive up;o Liebig's hones for him. Eventually, he nave un agricultural chemistry, except for the food aspects, turnihg h e a d to mineral and to public health chemistry. For example, he demonstrated the conditions under which lead piping could be safely used to carry Boston's supply of drinking water. His contraction of t v ~ h o i dfever in 1852 obviouslv scared him. Early deaths wiri by no means uncommon,"and he worried about the possibility of leaving his family unprovided for. He had never lacked ideas, although he often did not follow them uo. He becan to work on natentable items such as sulfite bleach, safety lamps, gas birners, and fertilizers. He seriously thought of resigning, hut stayed on until 1863, when he left to devote his full time to his Rumford Chemical Works in Providence, Rhode Island. There his extensive knowledge of phosphate chemistry yielded big returns through patented baking powder and medicinal beverages. Horsford seems to have been much more successful in industry then he ever was at Harvard. One of the earliest members of the American Chemical Society, Eben Horsford died in Cambridge, Massachusetts, on January 1,1893. An extensive account of Horsford's ancestry, character, and career has been given by Margaret Rossiter (9). Her book also contains a portrait of Horsford as he appeared in the late 1840's. Llferafure C k d 1. Nerbery,E. J. C h m Soc. 1318,113,701. 2. Gunning, H. E.:Gordan,R.A. J.Chem.Phys. 1942,10,126. a. Harsfard, E. N. Ann. derPhy8ik 1847,70,238. 4. Wheatstone, C. Phil. Tmna. 1843,133,303. 5. Schmidt, W.Am. delPhysik 1859,107,539, 6. Wiodemann. 6.Die Lehre "om G & v & ~ u s : Viewig: Braunnchweig. 1861: Vol. 1 n,L. Dos Leiluermagen der Eieklrolyf~;Teuhner:Leipzig
". -. ". F
8. Kohirauach. F.:Nippoidt. W. A. WLt. Noch. 1868,415: Ann. dm Physik l869,138.280, 17"
9. RarsiW. M. W . The Emergencoo/ Agrieullural Science: Yale University: NeuHaven. CT; Chapterel and 6.
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