SCIENCE AND PRACTICE. 297 



for removing the cable end from the bridge for a certain 

 time, and being able to replace it for the observation of charge 

 and discharge currents. The bar terminals T allow either 

 one-third, two-thirds, or the whole of the battery to be 

 brought into circuit. It remains now to arrange the board 

 for various cable measurements. 



43. Copper Resistance, when both ends of the Cable are 

 on the Board. One end of the cable to be measured is 

 attached to L, the other end to E, and a plug is put in 

 the hole L L X ; a plug is also put into the hole z l of the 

 battery commutator T, by which one-third of the battery 

 is put into circuit. The plug in hole I n of the tri- 

 angular commutator, U 3 , completes the upper branch, r r lt 

 without the constant element. The plugs of t^ are put 

 into the holes, so that either the zinc or the copper current 

 shall enter the cable, as may be required. In u 2 , also, two 

 plugs are inserted. In measuring the resistance of a cable 

 conductor, the lever of the key is kept down on its front 

 contact the whole time, and, whenever the resistance, R, is 

 altered, the plug between the slabs, G and G 1 (forming a short 

 circuit across the galvanometer coils), is inserted to prevent 

 the charge and discharge currents passing through and dis- 

 turbing the needle of the galvanometer. 



The current diverges into two circuits from the point c, 

 converging again in the point d into the main line, d, u l9 c. 

 The upper circuit includes so much of the resistance, r r 1 , as 

 is left unstoppered, the triangular commutator, u 3 , the point 

 of junction, a, the terminals L 1 , L, cable, E, and d. The lower 

 circuit is from c, through p, b, R, to c. Between c and d the 

 circuit is made up of TJJ, T battery, u x , K, &c. 



Having obtained a balance of the galvanometer needle, or 

 when the current in the circuit a, U 2 , G, U 2 , b is null, the pro- 

 portion 



r x 



~ 9 IT 

 or, 



X--RL - 



