404 



ME. J. J. MANLEY: OBSERVATIONS ON THE 



The resistance of one of the spirals being taken as unity, that of the other was 

 adjusted by trial until it was almost the same. The resistance of either spiral was 

 approximately 113 ohms. 



/"/> VII 



Two auxiliary coils, A, were now prepared. Each of these had a resistance ot 

 about 118 ohms, and consisted of 20 metres of insulated No. 35 German-silver wire. 

 The two wires were wound bifilarly and together upon a short piece of thick- walled 

 copper tubing, the surface of which had been covered with one layer of paraffined 

 paper. After winding, the coils were soaked in molten paraffin wax and then suitably 

 protected with silk ribbon and tinfoil ; the whole was then mounted upon a glass 

 tube, g, passing through the axis of the copper tube. Finally, binding screws were 

 soldered to the copper straps a, b, c, d, and e, and two equal copper wire leads, 

 twisted together, were used to join the straps, a, c, to the ends of a stretched Pt-Ir 

 wire.* The Pt-Ir wire had an effective length of 1 metre and a diameter of 1*5 mm. 



The following are the actual resistances of the four coils as determined at a 

 temperature of 20 C. : 



Pt coil marked L = 11277 ohms. 

 K= 112-99 



Auxiliary coil marked / = 117 '6 ohms. 



r= 117-6 



When in use the resistances of the two platinum wire spirals were adjusted to close 

 equality by adding 0'2 ohm to the external circuit of the coil L. 



It will be seen that the whole apparatus constitutes a Wheatstone's bridge of 

 dimensions convenient for the purpose in hand, and arranged after the manner devised 



* This wire was a portion of that used in a former research ('Phil. Trans,' 1898, vol. 191, p. 378). 

 It is remarkably uniform, and, when re-calibrated for the present investigation, was found to have a mean 

 resistance of 0-000155 ohm per 1 mm. 



