PEIRCE. — ELECTRICAL RESISTANCES. 



391 



and manganhie wire, wound on spools two inches in diameter over all, 

 and from four to eight inches long. The coils are furnished with stout 

 copper terminals (Figure 1), and are protected by cylindrical shields 

 made of brass or pasteboard tube. The copper terminals are screwed, 

 with axes parallel to each other, and one inch 

 apart, into one end of each spool. Spools and 

 shields together act as shunts to the coils, and it 

 has been necessary to determine a lower limit for 

 the resistance which such a shunt could offer in 

 practice. It has been necessary also to measure 

 the insulation resistance between the two mercury 

 cups, on a switch-board, into which the terminals 

 of one of the resistance coils dip. These cups are 

 formed by holes three eighths of an inch in diame- 

 ter (Figure 2), and five eighths or three quarters 

 of an inch deep. The axes of each pair of holes 

 are one inch apart. The holes are bored, or 

 drilled, in a lathe, in the top of the switch-board, 

 with a FiJrstuer bit if the material be wood or 

 vulcanized fibre or ebonite, with a flat or twist 

 drill moistened with water if slate or marble be 

 used. 



In order to get such information as I needed with respect to the 



insulation resistance which might safelj^ be counted on in the case 



of wood, marble, or vulcanized fibre, used for the purposes 



Ojust described, I made a long series of measurements, with 

 the help of a battery of twelve dry cells, and an absolutely 

 calibrated high-resistance mirror galvanometer, on a large 

 number of specimens. Of course individual measurements 

 of this sort have little general value, but a large number of 

 experiments on different samples of material of a given kind 



Omake it possible to set a lower limit to the resistance of 

 this substance when used in a given way, and such knowl- 

 edge as this is often useful when one is planning apparatus. 

 Fig. 2. For instance, my experience seems to show that it is per- 

 fectly safe to assume that the specific resistance per cubic 

 centimeter of an inch-thick slab of pure white Vermont marble, which 

 has been standing exposed to the air in a fairly dry room for three 

 weeks, is not less than 10® ohms, and is probably as much as 10^" ohms. 

 Eeally dry white marble has a far higher specific resistance than this. 

 It is safe also to set a lower limit for the specific resistance of sea- 



FlG. 1. 



