546 Drs. J. T. Bottomley and J. C. Beattie on 



means of an electric current. Fig. 2 shows the electric con- 

 nexions. A battery consisting of a sufficient number o£ 

 secondary cells was employed ; and the current was controlled 

 by means of suitable resistances, including a rheostat. In 

 the experiments here described the platinum wires of the two 

 tubes, the resistances, and the battery were all connected in 

 series, so that the same current passed through all*. 



The current in the circuit was measured by means of a 

 Kelvin ampere-gauge, and the difference of potentials at the 

 two ends of each of the platinum wires was measured by 

 means o£ a high-resistance reflecting-galvanometer. 



This potential galvanometer had a resistance of about 

 5000 ohms, and it was possible to insert in the galvanometer- 

 circuit an additional resistance of 10,000 ohms. 



In order to ascertain the absolute value of the readings of 

 the potential galvanometer, a standard coil of platinoid wire, 

 whose resistance was known very accurately, was joined into 

 the circuit, as shown in fig-. 2. This resistance was of con- 

 siderable length, and it was kept cool by being immersed in a 

 bath of oil. 



The following was the order of experimenting. The 

 pressure in the tubes was first reduced as much as possible by 

 means of the Sprengel pump ; then a very small current, 

 practically unable to heat any part of the circuit, was sent 

 through the two platinum wires and the standard coil, and the 

 potential-difference between the two ends of each was deter- 

 mined. This gave the ratio of the resistance of each of the 

 platinum wires to that of the standard coil, all being cold, and 

 at the same temperature. The current from the battery of 

 storage-cells was now suitably increased, and readings were 

 taken in the following order : — The current passing was first 

 read, llien the zero of the potential galvanometer was noted, 

 and the deflexion of the potent : al galvanometer when con- 

 nected to the two ends of the standard coil was observed. 

 The electrodes of the potential galvanometer were next 

 applied to one of the platinum wires, and the deflexion noted; 

 then the deflexion due to the second wire was observed. A 

 second reading was taken from the first wire and also from the 



* In another set of experiments the platinum wires were jo'ned in 

 parallel, and, by means of two rheostats, one connected in series witli 

 each platinum wire, an attempt was made to regulate the current in each 

 wire so that the temperatures in the two should be the same. This was 

 found very difficult to carry out ; but it is intended to renew the 

 attempt r and determine simultaneously the radiation from two wires 

 with different surfaces, in the same vacuum, and at the same temperature. 



