PEIRCE. 



ELECTRICAL RESISTANCES. 



393 



of an air-pump, and the air be alternately exhausted from and re- 

 admitted to the receiver, the resistance of the conductor is always 

 materially increased. This process is in common use for the purpose 

 of improving the insulation between the layers of covered wire in 

 galvanometer coils. A coat of shellac, when not thoroughly dry, often 

 lowers very much the insulation resistance of a porous half-conductor. 

 The followiuij table shows the results of measurements of the resist- 

 ance between the two members of each of a large number of pairs of 

 mercury cups of the size shown in Figure 2 bored in the tops of slabs 

 of different substances. The specific resistance of wood for currents 

 going across the grain is generally from 20 to 50 per cent higher 

 than for currents going with the grain. The figures given below may 

 be taken as referring to currents going with the grain. I procured, 

 wherever I could conveniently, a number of pieces of seasoned wood 

 of each variety named, rejecting none, and I believe that the numbers 

 in the table represent fairly what one may expect in practice. The 

 number in the second column of the table, in the same horizontal line 

 with the name of a substance, shows how many pairs of mercury cups 

 bored in slabs of this substance were experimented on, while the 

 numbers in the next two columns give in megohms the lowest and 

 the average resistance between the members of these pairs. Some- 

 times a single pair of cups only was bored in a slab, sometimes two or 

 three. The resistances between the members of pairs of mercury cups 

 bored in the single specimens of cypress and maple that I had were, in 

 the average, upwards of 2,000 megohms. 



TABLE I. 



