N e w electromagnetic weighing balance developed
OnERN techniques in science and M technology often require unusual and unconventional approaches to the classical problems of precise weighing. Electromagnetic u-eighing i? an ancient art hut its possihilities become increasingly attractive largely as a consequence of the unremitting advances in electrical measurement. The very earliest studies of Emirh in quantitative inorganic microprocedures made use of Nernst's electromnenetic halance [Nernst, W.,Gollinge;~'hTach7ichln,pp, 75-82 (1903), Nernst, W.,Rienenfeld, E. H., Ber. 36, 2086 (1903)l. Related techniques were employed by Lamb in very precise determination of density. The earlier methods, as a rule, required
empirical calibration hut there is no sound reason why this should he absolutely necessary and indeed slight nonlinearities can be accommodated in properly designed circuitry. Some years ago, a British development was described in this column in which the pointer and coil of a D'Arsonval movement was used as a torquemeasuring element. The principle vas extended to weighing in the milligram and microgram range by the simple expedient of applying the load to the pointer and then passing measured currents through the coil to counterbalance the load. Completely automatic rebalancing as achieved electronically by sensing the needle posit,ion capacita-
A Figure 1. balance
General view of Electro-
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MAGNETIC FIELD
H
b Figure 2. Schematic drawing of Electrobalance VOL. 29, NO. A, APRIL 1957
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49 A
.,. INSTRUMENTATION
instrument abstracts
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I
Applied Physics Corporation/Pasadena/California
For Ultraviolet-Visible Spectrophotometry
The Cary Model 11 provides performance comparable to the finest; cost comparable to the cheapest.
a Figure 3. partment
Details of weighing com-
tively and providing a feed-hack loop to supply the load-balancing current. Obviously, this system could provide a continuous or recorded record of changes in weight as a function of time or en1-ironment. New Elcrtrommgndic Weighing Balmnce
(2100 & . to 8000 A) the Cary Model 11 provides the same high degree of accuracy, ruggedness, and dependabilitv as the Model 14. as well as its
Considering t h e purchase o f a recording spectrophotometer? The following comments may help YOU get much more for your money.
cover t h a t quite a few people Light: Less than 0.0001% over do not realize t h a t t h e hlodel 14 Strmy most of the range. has a companion instrument-the Scanning Speeds: 1.0 8, per second (ultraviolet region) to 125 A per Cary hlodel 11-which gives t h e second (visible region). same high quality .of the \lode1 14 Resolution: 1.0 A or better throughout at n cost compnrablc to the lowestmost of thc range. price recordiug spectrophotometcr. Wavelength Acrurcxy: Better than 5.0 A in the ultraviolet region and better The difference between the Model 11 than 10.0 A in the visible r e a m . and the Model 14 is io wavelength Reproducibility: Better than 0.5 A in range. Of course, where the wider the ultraviolet and 3.0 A in the viswaveleneth ranee is reouired. the ihle r ~_ nc i n n ,, . . .- _ Model 14 i s the finest instrument Photometric Reproducibility: Repraducib i~ b hctter~than .004 a v a i ~ n ~ )H ~e. ~ for ~ ~ ~ in absorbance ~ , can be achieved with the .Model 11. in the ultraviolet and visible ranges 20,000 HOURS OF SERVICE WITHOUT MAJOR REPAIRS
The first Cary Model 11was pmduced in 1947, and since thcn nearly every Icnrling analytical Iahoratory in the United States-and many abroad-has acquired one or more hlodel 11’s. The performance, flexibility and reliability of the Model 11 have h e n proved in all kinds of research and c o n t d applications. One of the first instruments to he manufacturec-Serial No. 2-was recently overhauled at the factory after having been in use twelve hours Per day for over six years without rewiring any
service other than routine maintenance. This instrument has now begun a second stint of reliable service which will undoubtedly run into additional thousands of hours. FREE BULLETIN
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i f your spectrophotometer applications are in the visible or ultraviolet range, investigate the many advantages of the Cnry hiodcl 11. For complete infomation write to Applied Physics Corporation, 362 West Colorado Street, Pasadena 1, California, for Bulletin ~ ~ 2 4 .
For further infomatim, circle number Sod P I Readers’ SeWiCl Card. V W 17 PI
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ANALYTICAL CHEMISTRY
An instrument employing some of these principles has been developed by the Cahn Instrument Co., 7712 Danvers St., Downey, Calif. It is manually operated, although a recording version is contemplated. A general view of the Electrobdance is shown in Figure 1. It is rugged, unaffected by shock, vibration, or temperature change and is portable. The dimensions are 10 X 10 X 7 inches and it weighs less than 12 pounds. The weighing compartment is in the upper right and the only essential connection between it and the rest of the instrument is electrical. The beam, stirrup, and pan are made extremely light and these protect the jewel bearings in the electromagnetic torque element from severe acceleration forces. The circuit has been designed for speed and convenience as a-ell as a?curacy. In use, the pan, calibrating mass, and sample weights are sncce4sively balanced by controlling current through the precision torque motor. The zero control “remembers” the cnrrent for zero weight, and the calibrate control “rememhers” the resistance needed to make the dial read directly in micrograms. In making a weighing, the beam is balanced with the balance control. The galvanometer (upper left) is brought to null position with the mass dial, from which the mass is then read directly. These operations require 20 to 30 seconds. A schematic of the Electrohalance is shown in Figure 2. The pointer scale, behind which the pointer moves, is illuminated from the rear. Weight ranges are selected by means of a panel cont,rol. A larger view of the weighing compartment is shown in Figure 3.
Model IL-9 Capacity: 120 gm.
Model LA-3 Capacity:
200 gm.
Model IL-lI Capacity:
4'/2
Kg.
*Sensibility Reciprocal:
*Sensibility Reciprocal:
10 mg.
260 mg.
Model RX-1 Capacity: 120 gm.
*Sensibility Reciprocal:
*Sensibility Reciprocal:
20 mg.
4 mg.
Model 1L-7 Capacity:
2 kilo (5 Ibs.)
*Sensibility Reciprocal:
200 mg. 'Sensibility Reciprocal is the amount of weight required to change the rest posltlon of the indicotor one full tcole division. For fwtber lnfomrtlon, circle uumber 61 A on Readers' Senice Card, page 77 A VOL. 29, NO. 4, APRIL 1957
28 f4b
51 A
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INSTRUMENTATION
A P I E Z O N OIL DISTILLATES for H I G H VACUUM W O R K These Apiezon products have the recognition of leading scientists who a c c o r d them top preference f o r h i g h vacuum work. Because of the exceedingly low vapor pressure of all these oil distillates, at usual room t e m p e r a t u r e s , they are in great demand for all sorts of h i g h vacuum work. Their vapor pressures are a s l o w as mm. Hg, a n d even unmeasurable a t room temperature. As the exclusive distributor in the United States, we offer immediate delivery from stock on the complete range of these oils and compounds. For applications, working characteristics and list prices, write for Bulletin 43-AC.
LEYBOLD MERCURY EJECTORS and DIFFUSION PUMPS T h e Model Hg 3 mercury diffusion p u m p reaches its full pumping speed at pressures below 10-2 m m . H g and remains constant down to mm. Hg. T h e mercury ejectors models H g 12 and H g 45 'reach maximum pumping speed a t pressures between 10-1 a n d 1 0 - 2 mm. Hg. Specially designed jets permit constant pumping speed at pressures between 10-5 a n d 10.~ mm. Hg. The pumps begin to operate at considerably hieher messures. T h e ultimate vacuum is better t h i n 16.6 m m Hg. W e welcome your inquiries on this equipm e n t and t h e accessories for it. Write for Bulletin 44 AC.
Figure 4. Separate component design focilitotes remote weighing
Remote Weighing. Po*sibls
The Electrobalance is particularly suited for remote operation in hot radiation cells, dry boxes, or other inaccessihle environments because, aside from adding or removing samples, the weighing operation is performed electrically and can he done remotely. Figure 4 illustrates the ease with which the balance compartment can be removed and connected by a four-terminal lead to the measuring system. It is also possible to monitor a number of weighing asscmhlies from one measuring and control box by suitable switching means. The conventional microbalance, of course, covers a much wider range and with high precision. According to Niederl, such a balance, suitable for quantitative organic elementary analysis, will he reproducible to f 2 y at a 1-gram load, 1 5 y for 1- to lC-gram loads, and no more than + I O y for 10to 20-gram loads. Nevertheless the Electrobalance in a limited range can be very useful for rapid, small-sample weighings. An indication of the range and precision is given by the manufacturer as follows: Range of
Sample Sensitivity, Weight, Mg. Y 0-5 0-10
wzn 0-50
Accuracy,
5
Y 2.5 5 12
8
30
1
2
a precision of about 0.02% of full scale and an accuracy of about 0.05'%. The ease and convenience for small sample weighing would seem to be verv useful in the direct weighing of small samples for the KBr pellet technique in infrared work, for d.c. arc hollow electrode technique in emission spectroscopy, and in the direct preparation of solutions foi ultraviolet spectrophotom?try.
