[RUTHERFORD] DISCHARGE OF ELECTRICITY FROM PLATINUM 29 
found that the slope of potential was extremely small near the heated 
plate and was greatest neur the plate B. If the ionization is intense, we 
can thus make B zero without any appreciable error. 
Then a pe ax? 
dx 
Integrating between the limits 0 and d, 
= 2 12 
v= — gad 
Sni 3 
or v= — ay K d 
Ae sy al 
inl Uy Seo ee 
à 327 d; 
The ionization theory, applied to the question of the current between 
two parallel plates, when the ionization is powerful and confined mainly 
to the surface of one electrode, thus leads to the following conclusions :— 
1. The current is independent of the intensity of the ionization, /.e., 
of the temperature of the plate beyond a certain limiting 
value ; for the equation, giving the current, does not involve 
the intensity of the ionization at the surface of the plate. 
2. The current is directly proportional to the square of the voltage 
between the plates. 
3. The current is directly proportional to the velocity of the ions 
and inversely as the cube of the distance between the plates. 
A brief account will now be given of the experimental arrangement 
and some of the results obtained in verification of the theory. 
The platinum foil which served as a source of ionization was rect- 
angular in shape, 14 cms. long, 7-5 cms. wide, and 002 ems. thick. It 
wus fastened at each end to solid copper plates by means of silver solder, 
The upper plate was attached to a copper rod through which the current 
entered and the lower plate dipped freely into a mercury trough. The 
foil was thus always in a vertical plane and no correction was necessary 
for sag of the foil with rise of temperature. 
The heating current (up to 160 amperes) was supplied by a small 
motor-generator and regulated by a rheostat, The current was measured 
by a Western millivoltmeter off -0001 ohm standard. The whole 
system was moderately well insulated and could be charged up to 600 
volts above the earth, by means of a battery of small accumulators, one 
pole of which was connected to earth. 
The current due to the ionization at the surface of the heated plat- 
inum plate was measured by means of a very sensitive galvanometer 
between the platinum plates and two vertical copper plates placed at 
equal distances from opposite sides of the platinum foil. These copper 
