abstracts: physics 85 



two, and only two, special electrical instruments. The first is an ar- 

 rangement for eliminating the effect of parasitic thermal electromotive 

 forces. A common copper knife-switch will perform this service ad- 

 mirably. The second special requirement is an appropriate potenti- 

 ometer, that is, one reliable to 0.1 microvolt. 



The slide-wire 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 potenti- 

 ometers, or potentiometers having two otherwise separate, very simple 

 instruments in series in the same galvanometer cu-cuit. A very low- 

 priced split-circuit potentiometer is on the market; and the combination 

 potentiometer, on account of its mechanical and electrical sunplicity, 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 dif- 

 ferently varying electromotive forces. Its convenience for such meas- 

 urements 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-switch, can be arranged 

 at the cost of a few knife-switches and very little labor. An especially 

 suitable instrument to arrange in this way is the combination potenti- 

 ometer, all of whose dial switches are single, and free from contact re- 

 sistance error. 



Another convenience especialty easy to obtain with the thermo-ele- 

 ment-potentiometer system is the power to take the last two figures of 

 any reading directly from the galvanometer scale. It increases speed, 

 simplifies manipulation, dimmishes errors, and gives calorimetric data in 

 a form specially convenient for further treatment. 



For high-temperature measurements and much other thermo-element 

 work not calorimetric, though the required precision may often be less, 

 most of the features above described are desirable, especially the facility 

 for simultaneous and direct readings. W. P. W. 



PHYSICS. — Leakage prevention by shielding, especially in potentiometer 

 systems. Waltee 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 



