452 Prof. D. E. Hughes. [May 27, 



Fig. 1. 



A, B, 0, D are the four sides of the bridge, the telephone J 

 replacing the ordinary galvanometer; the sides CB and CD are 

 formed of German silver wire, each 50 cm. in length, 0*5 mm. in 

 diameter, and 0'85 ohm resistance ; the sides AB and AD have also 

 0'85 ohm resistance, so that the four sides have equal resistance, 

 and this remains a constant during the whole series of experiments. 



AE and AF is a continuous spiral of copper wire, formed of hard 

 copper silk-covered wire of 1 mm. diameter and 4'80 metres in 

 length, wound loosely on a boxwood cylinder of 3'50 cm. diameter 

 and 30 cm. in length, on which it moves freely ; the entire helix 

 has 40 turns of 4 cm. diameter, each spiral being separated 5 cm. 

 from each other; the spiral, however, is separated into two equal 

 portions by giving a greater separation at the centre in order to 

 allow it to be attached to a sliding collar of wood, by means of which 

 we can press the spiral closer together on either side as desired ; at E 

 and F there are also adjustable collars of wood, and as the boxwood 

 cylinder is graduated, we may (if the central collar is fixed) approach 

 more or less either side of the helix, and read the degree of approxi- 

 mation of the coil, thus adjusting the mutual induction of each side 

 to a perfect balance or zero. In practice I prefer moving the central 

 collar, using the end collars only for the perfect adjustment of its 

 zero, as we then have a double effect, viz., closing the coil say from 

 A to F, increasing its mutual self-induction, and at the same time 

 decreasing the previously balanced induction of the coil AE ; this not 

 only gives a wider range of effect, but renders the scale readings 

 more uniform ; the end F of the helix is joined to about 10 cm. of 

 German silver wire, completing the circuit from G to D ; this supple- 

 mentary German silver wire is simply for the purpose of making the 

 resistance of AD equal to DC, and its length should be adjusted to 



