298 THE ELECTRIC TELEGRAPH. 



is established. The values which this proportion may have 

 are, obviously 



1. When r p 



r 10 100 1,000 



may be = , , or 



p 10 100 1,000 



2. When 



10 



, , or . 



p J 100 1,000' 10,000 



The limits within which these proportions are confined are 

 therefore, ^r = 1,000, and J-QQQ = 0,01 ; and, since the 



greatest value of the adjustable resistance, R, is 10,000 units, 

 and the least value one unit, the limits of the measureable 

 value of x with the bridge system so arranged, are 



Maximum : x = 10,000 X 1,000 = 10,000,000 

 Minimum : x = 1 X 0,001 =: 0,001. 



Whenever it is possible to do so, it is to be preferred to 

 make the ratio = 1, and r as nearly equal to R as can be, 



by which the greatest sensibility of the system is obtained. 



44. Copper Resistance, when only one end of the Cable is on 

 the Board. The connections of the board remain the same, 

 except that, instead of the second end of the cable being con- 

 nected to E, it is connected to earth. The upper circuit is 

 then from c through r, U 3 , a, LJ, L, cable and earth ; and the 

 lower from c through p, b, R, d, and earth. 



In measuring the copper resistance of a cable with earth - 

 plates in the circuit, it is necessary (1) to have the plates, both 

 of the apparatus and cable-end, sufficiently large, and buried 

 in moist ground, or sunk in water to avoid the introduction 

 of resistance into the cable circuit ; and (2) to have both the 

 plates of the same metal, and of the same quality of metal, at 

 the same temperature, and, as nearly as possible, in water of 



