when an Electron escapes from Surface of a Hot Body. 203 



system in either of two ways. In the first method it was 

 introduced at the middle point of the arm CF by means of 

 the high-resistance shunt PQ. In the second, the shunt 

 was placed around the arm FE by means of the key K a . 

 With this arrangement the thermionic potentials w r ere 

 applied to the middle point of the filament L. The 

 thermionic potentials were measured by means of the micro- 

 ammeter, T. This instrument was a Paul testing set, giving 

 1 scale-division per 2 microamperes and reading up to 

 375xl0~ 4 ampere by means of suitable shunts. It was 

 calibrated by means of a Wolff potentiometer and a Wolff 

 standard resistance. The thermionic potentials were supplied 

 by a 120-volt storage battery, across which was shunted a 

 1500-ohm slide-wire resistance from which suitable potentials 

 were tapped off. The potential used was ordinarily about 

 40 volts. The thermionic currents were started or stopped 

 by reversing the commutator K 2 . The galvanometer used 

 was of the d'Arsonval type, with a resistance of 225 ohms, 

 and gave a deflexion of 1 scale-division for a current o£ 

 9 x 10~ 9 ampere. 



Method of Observation. 



Observations were carried out as follows. The key K 3 

 was closed, so as to shunt the 10-ohm standard in the 

 arm CF. With the thermionic current off, K 4 was opened 

 and closed and the resulting galvanometer deflexions were 

 recorded. Usually seven deflexions constituted a set. Then, 

 with K 4 open and K 2 alternately reversed, the resulting 

 deflexions due to turning the thermionic current on and off 

 were recorded. From seven to fifteen deflexions were taken 

 in this case, depending on the magnitudes of the deflexions 

 and the steadiness of the galvanometer. The mean de- 

 flexion was determined in most cases by the method 

 described in Richardson and Cooke's first paper*. The 

 ammeters A and T were read, the voltage of B 1 determined, 

 and the resistance of L w 7 as read from the bridge system. 

 K x was now reversed, and the whole system of measurements 

 repeated. The two series of observations constituted one 

 complete set for the " Standard Shunted " arrangement. 

 An exactly similar complete set was taken with K 3 closed, 

 so as to shunt the filament instead of the 10-ohm standard. 

 This second set with the " Filament Shunted " arrangement 

 gave the data for calculating an independent value of <j>. 

 The two methods gave consistent results. 



* Phil. Mag. July 1910. 



