Properties of the Alloys of Cast-iron and Aluminium. 607 



in Part L (Phil. Mag. Jan. 1900). The four arms of the 

 bridge consisted of 



(1) The thermometer-wire and leads. 



(2) The compensating leads, a resistance r 2 , and a portion 

 of the bridge- wire EH. 



(3) A resistance of 275 ohms. 



(4) A resistance of (275— p) ohms + portion of bridge- 



wire HF. 



[p being equal to the resistance of H F to within ^~ of an 

 ohm.] 



When a balance is obtained, the resistance r t of the ther- 

 mometer-wire is given by 



rt = r 2 + resistance of E H. 



The temperature was deduced from r t by means of the 

 formula 



'="»S +8 {(ro)-»}- 



The values of r Q , r 10 o, and S for the given specimen of wire 

 were determined from observations of its resistance in ice, 

 steam, and the vapour of boiling sulphur. 



This specimen of wire was obtained from Johnson &Matthey, 

 and was of the quality prepared by this firm for thermometer 

 work. 



For this wire 5 = 1*57 



and riw=r Q (l + 0*377). 



§ IV. The Heating Circuit. 



The heating circuit consisted of a platinum wire, of dia- 

 meter 0*5 mm., well insulated with asbestos-paper, and wound 

 non-inductively round the ring outside the other coils. This 

 wire was connected with an ammeter A (vide fig. 1), an 

 adjustable resistance N of thick platinoid wire, and the 

 terminals of the town mains (100 volts) P, L. By altering 

 the value of the resistance N the heating current could be 

 varied at will, and thus any required temperature (within 

 the limits of the experiments) could be obtained. 



With this arrangement a current of 7 amperes raised the 

 temperature of the ring above 800° 0. 



The insulation resistance between the various coils was 

 tested from time to time during the experiments. In no 

 case (after the first heating) was it less than 1,000,000 ohms, 

 and in general it probably was much greater than this. 



