PRINCIPLES OF DESIGN. 67 



on the slide wire by a cell with key and resistance in circuit. 

 To one end a high resistance galvanometer G is permanently 

 connected. The wires C D are connected to the terminals of 

 the standard wire, and the movable contact on the slide wire 

 shifted until balance is obtained. This is shown when there is 

 no deflection on the galvanometer, the voltage on the slide 

 wire between and the sliding contact being exactly equal 

 and opposite to that at the terminals of the standard wire. 

 The same is repeated for the test wire, the wires C D being 

 transferred to the test-wire terminals. The reading in each 

 case of the pointer on the slide wire gives the relation of the 

 voltages and therefore of the resistances of the standard and 

 test wires, from which the resistance of the test wire is obtained. 

 The voltages dealt with are very small, but this null method 

 with a highly sensitive galvanometer is capable of giving very 

 accurate measurements of low resistances. 



The standard wire may be a wire of 100 per cent, conduc- 

 tivity equal in length and weight to the test wire or in the 

 form of a coil of equivalent resistance at the same temperature. 

 For example, say the test wire is 30ft. of No. 18 copper wire 

 weighing 1,465 grains. The resistance of a standard coil 

 equivalent to that of 30ft. of pure copper of the same weight 

 would be found thus : — 



9:^1^^J^ = 0-U28 standard ohms 

 1465 



at 60°r., where 0*2162 is the resistance in standard ohms at 

 60°F. of a piece of pure copper 1ft. long weighing one grain. 

 Suppose now that the potentiometer slide readings were 690 

 and 670 for the test-wire and standard resistance respectively, 

 the resistances would be in the same proportion and the con- 

 ductiivity of the test wire would be 



1^x100 = 97 per cent. 



Gutta-percha and vulcanised india-rubber are chiefly used as 

 the insulating materials in submarine cables, but pure india- 

 rubber is also used to some extent. The chief advantage that 

 gutta-percha has over india-rubber is in its non-porous and non- 

 absorbent nature, and in the facility with which, when heated^ 

 it can be laid on the conductor and drawn through dies 



f2 



