SCIENCE AND PRACTICE. 363 



R being the amount of unstoppered resistance in the set of 

 c oils between and u v r f the resistance of a, and r" that of 

 /3. An advantage is derived from the measurement of the 

 magnetic constant of the coils by the galvanometer with a 

 single battery, as no error can occur from inequality in the 

 electro-motive forces. 



For the measurement of insulation resistance, the cable 

 is connected with the terminal u 2) in which the contact 

 plug is inserted, so that the cable end is, in fact, connected 

 with the beam K 3 of the key, or that which is the same thing, 

 with the upper end of the coil a. Contact plugs are inserted 

 in all the holes of r and p ; diagonally in u 2 , so as to put the 

 copper pole or the zinc pole of the battery B to earth as occa- 

 sion may require ; in u 3 , so as to introduce a sufficiently great 

 battery power to make the observation sensible, when 1 per cent. 

 of the electro-motive force is added or subtracted; in u, in holes 

 1 2 and 4 5 ; in U! in hole i in ; in the hole of u 2 , when 

 the moment arrives for sending the current of B into the 

 cable ; and lastly, after the expiration of a certain time, the 

 key is pressed down, by which the short circuit is broken 

 (letting the insulation current go through , and the opposite 

 current of b through ft), and R is closed at the same instant. 

 In the changed position of the stoppers of u, the resistance 

 coils R, which were in the circuit of a, in measuring the con- 

 stant, are now shifted into the circuit opposite. The current 

 of B goes through earth, u 2 , u 3 , u 2 , r, a, u 2) cable, dielectric, 

 earth. The current of b goes through the circuit, u t (in, i), 

 key (1, 3), ft /, u (2, 1), R, u (5, 4), b, &c. 



The resistance of R is adjusted until the needle is not de- 

 flected to either side of zero. The resistance of the cable is 

 then expressed by 



r 1 the resistance of a is so small, in comparison with #, 

 that it may, without sensible error, be neglected, and the 

 resistance of the cable be called 



