Properties of the Alloys of Iron and Aluminium. 129 



wires of the same resistance as those connected to K 4 joined 

 these cups with the key K 3 . 



The object of the oil-bath was to maintain the junctions of 

 platinum and copper at the same temperature, and thus get 

 rid of thermoelectric currents. 



The leads were protected from direct heating by the furnace 

 by means of a thick sheet of asbestos. 



The resistance-bridge H L M J was so arranged that the 

 resistance of the secondary could be obtained directly from 

 one observation. 



This end was gained by the following device : — 



The resistance of the arm HJ was made equal to 1001 

 ohms. 



The resistance of MJ consisted of three parts, r 2 , r 3 , and p 2 . 



r 2 was a resistance of 1000 ohms ; r s consisted of coils of 

 tenths of an ohm ; and p 2 was part of the bridge-wire nd. 



The resistance of the whole wire nO was slightly greater 

 than 1 ohm. Hence, by taking the proper plugs out of r s , 

 the resistance of p 2 and r 3 could be made together equal to one 

 ohm to within one-twentieth of an ohm ; i.e., the resistance of 

 the arm MJ is equal to 1001 ohms to within a twentieth of an 

 ohm. To obtain a reading, the secondary and galvanometer 

 are switched on to the wire-bridge by means of the keys K 4 

 and K 2 , and r 4 and p 1 are adjusted until there is a balance 

 [withMJ%HJj. 



When this is the case the resistance of the secondary is 

 given by 



n = n+pi> 



The compensating leads are connected to the arm LM by 

 the key K 3 . 



The resistance of different lengths of the bridge- wire having 

 been previously tabulated, the resistance of the secondary could 

 be read off directly from the scale. 



The temperature t of the ring of alloy was obtained from r, 

 by means of Callendar and Griffiths's well-known formula 



r 100 , r being the values of r at 100° C. and 0° 0. respectively, 

 and 8 having been obtained from observations in the vapour 

 of boiling sulphur. 



[To obtain the resistance of the secondary and leads 

 together, the mercury cups 2 and 4 in the key K 3 were joined 

 by a thick copper wire. Then we have, when balance has 

 been obtained as before, 



r t + leads = r( + p{.~\ 

 Phil. Mag. S. 5. Vol. 49. No. 296. Jan. 1900. K 



