288 
Mr Richardson , On the Negative Radiation 
than 1'5 amperes ; it was found that the temperature of the wire 
was practically constant during each observation ; the variation 
(about 5°) being due to gas given off from the wire. The tem- 
perature of the wire therefore steadily decreased during each 
observation, but it was held that by taking the mean of readings 
immediately before and after each measurement of current from 
the wire the true temperature would be obtained. The resistance 
was measured by placing the wire in one arm of a Wheatstone’s 
bridge, the corresponding arm being a thick German silver 
resistance of 1*7 ohms. This resistance was not sensibly heated 
by the current which passed through it. The resistance of the 
other two arms was of the order of 1000 ohms, so that practically 
the whole of the heating current passed through the wire and the 
German silver resistance. The heated platinum wire passed 
axially through the aluminium cylinder, being fixed to two 
electrodes in the bulb which contained the cylinder. The bulb 
was connected with a drying apparatus, pump and McLeod gauge. 
In most of the experiments the pressure was about '02 mm. but 
it varied from *01 to T6 mm. It was very difficult to keep the 
pressure down at the higher temperatures owing to the gas given 
off by the hot wire. 
The same Thomson galvanometer was used both to measure 
the leak and for the Wheatstone’s bridge. It had a resistance of 
4058 ohms and gave 1 scale division for a current of 7 x 10 -10 
amperes. In order to use it for both purposes, suitable shunts 
had to be inserted. The whole of the measuring apparatus was 
insulated in paraffin so that the potential of the hot wire could be 
raised to any multiple of 40 volts up to 400. It was found that 
with 400 volts positive on the wire, there was no observable 
deflexion of the galvanometer, whereas quite big currents were 
obtained when the wire was negative. In all cases the saturation 
current, i.e ., the total number of ions given off by the wire, Avas 
measured. 
To reduce the determinations of resistance of the platinum 
wire to temperatures use was made of the measurements of the 
melting points of potassium and sodium sulphates by Messrs 
Hey cock and Neville 1 . The wire was set up in air and its re- 
sistance determined first at the ordinary temperature, and after- 
wards when the smallest possible grain of potassium sulphate 
placed on it just melted. In this way the resistance for two 
temperatures differing by about 1000 degrees was obtained, and 
the temperature corresponding to any other resistance reading 
could be got by interpolation from the curves given by Professor 
Callendar 2 . To test the method, the melting point of sodium 
1 Heycock and Neville, Chem. Soc. Journal , lxvii. 1895, p. 160. 
2 H. L. Callendar, Phil. Mag. xlviii. 1899, p. 519. 
