152 CARNEGIE INSTITUTION OF WASHINGTON. 



(33) Therino-element installations, especially for calorimetry. Walter P. White. J. Am. 



Chem. Soc, 36, 1856-1S68 (1914). 



(34) Potentiometers for thermo-electric measurements, especially in calorimetry. Walter 



P. White. J. Am. Chem. Soc, 36, 1868-1885 (1914). 



These two papers describe a type of auxiliary installation for thermo- 

 elements which in high-temperature measurement and other work of moderate 

 precision is valuable for its convenience, quickness, and comprehensiveness, 

 and which is also capable of the very high precision often desired for calorimetry. 



When a thermo-element is used with its two ends at nearly the same temper- 

 ature, a condition easily provided in calorimetry, the relative precision required 

 in the electrical measurement falls to a value no greater than that desired in 

 the temperature reading, and the most serious errors ordinarily affecting the 

 electrical thermometer practically disappear. 



The absolute electrical precision required is also comparatively low. With 

 a convenient and easily made copper-constantan multiple thermo-element of 

 24 couples, 0.0001° C. corresponds to 0.1 microvolt. 



The satisfactory attainment of a precision of 0.1 microvolt demands two, 

 and only two, special electrical instruments. The first is an arrangement for 

 eliminating the effect of parasitic thermal electromotive forces. A common 

 copper knife-switch will perform this service admirably. The second special 

 requirement is an appropriate potentiometer, that is, one reliable to 0.1 micro- 

 volt. 



The slide-Avire and Feussner potentiometers of 1 volt range or more now in 

 common use are not thus reliable, and are otherwise unsuited for thermo- 

 element work. Split-circuit potentiometers are satisfactory in this and all 

 other important respects, and so are combination potentiometers, or potentio- 

 meters having two otherwise separate, very simple instruments in series in the 

 same galvanometer circuit. A very low^-priced split-circuit potentiometer is 

 on the market; and the combination potentiometer, on account of its mechan- 

 ical and electrical simplicity, is an easy instrument to build to order. 



The potentiometer system, either with or without the thermo-element, is 

 especially suited to simultaneous measurements of different and differently 

 varying electromotive forces. Its convenience for such measurements can be 

 increased by using a few pieces of hard-rubber sheet as stops for the dial 

 switches, and still further increased by arranging a double potentiometer, with 

 duplicate dials. One effective form of double potentiometer, which employs a 

 master-s\vitch, can be arranged at the cost of a few knife- s^\'itches and very 

 little labor. An especially suitable instrument to arrange in this way is the 

 combination potentiometer, all of whose dial switches are single, and free from 

 contact resistance error. 



Another convenience especially easy to obtain with the thermo-element- 

 potentiometer system is the power to take the last two figures of any reading 

 directly from the galvanometer scale. It increases speed, simplifies manipu- 

 lation, diminishes errors, and gives calorimetric data in a form specially con- 

 venient for further treatment. 



For high-temperature measurements and much other thermo-element work 

 not calorimetric, though the required precision m.ay often be less, most of the 

 features above described are desirable, especially the facility for simultaneous 

 and direct readings. 



(35) Leakage prevention by shielding, especially in potentiometer systems. Walter P. 

 White. J. Am. Chem. Soc, 36, 2011-2020 (1914). 



An insulation resistance of 5,000 megohms or more is often necessary to 

 prevent serious disturbance of thermo-electric measuring systems from stray 

 portions of power or lighting currents, and the frequently more sensitive 



