ME. O. W. RICHARDSON ON THE ELECTRICAL CONDUCTIVITY 
olO 
Time of observation. 
Current observed 
(in scale divisions). 
12.18 p.M. 
183 
12.34 „ 
191 
12.43 „ 
201 
12.56 „ 
218 
1.15 „ 
205 
3.16 „ 
208 
3.48 „ 
193 
11.5 A.M. (next day) 
193 
This mode of observation is jDractically that followed in § 5, where the current is 
observed immediately after the temperature of the wire has been increased by a given 
amount. 
§ 3. Experiments ivith Alternating Currents. 
A mode of observation which is especially well calculated to show the relation 
between the positive and negative ionisation produced by hot j^latinum in a vacuum 
is to heat the wire by putting it on a 200-volt alternating circuit and to observe the 
current to the cylindrical electrode. The ions of both signs are alternately driven 
away from and attracted to the hot wire owing to the alternating field between the 
wire and the cylinder. The cylinder is connected to one quadrant of an electro¬ 
meter, the other quadrant being put to earth. The directioii of the current, which is 
indicated by the direction in which the spot of the electrometer moves, is determined 
by the sign of the ions which reach the cylinder in greatest quantity under the alter¬ 
nating electromotive force. At low temperatures all the ions produced by the wire 
are positive, so that the current is necessarily in the positive direction. At higher 
temperatures negative ions are also produced in gradually increasing quantity, so that 
at one temperature the same number of positive and negative ions reach the cylinder 
in a given time. At this temperature, which may be called the transition tempera¬ 
ture, there is no current from the hot wire to the surrounding electrode under the 
given alternating field. At still higher temperatures, owing to the rapid rate at 
which the negative ionisation increases with the tenq^erature, the current is always 
negative. 
In these experiments the temperature of the wire was not determined, but a rough 
idea of it can be obtained from the resistance. This was determined with the 
apparatus indicated in fig. 2, except that the galvanometer G was replaced by 
a telejohone. Tlie following table gives corresponding values of the leak and the 
resistance of the wire :— 
