Discharge of Electricity. 



399 



in a brass plate b by 



wire p p (fig. 2) were insulated 

 means of vulcanite plugs v, 

 and placed so as to be in Fig. 2. 



planes at right angles with 

 one another and with the 

 brass plate. The shortest 

 distance between the wires 

 was 4 millims. The figure 

 represents the apparatus sus- 

 pended by means of the 

 upper wire from one con- 

 ductor of the Holtz machine, 

 and the lower wire connected 

 with the positive conductor. 

 Only the wire which at the 

 time was not bearing the 

 weight could be heated with- 

 out changing the distance of 

 the wires. The wire operated 

 on was made red-hot by 

 bringing the terminals of a 

 battery of four Bunsen's elements into contact with it at two 

 points one inch apart ; and the charging was made as soon as 

 possible after the heating. Before heatirig, the spark re- 

 quired a difference of potential of 125; after heating, a mean 

 difference of potential of 93. This was in the case of a single 

 spark. When a continued spark was taken, a similar dimi- 

 nution of the deflection was observed. Here the effect could 

 not be due to a rarefaction of the air between ; it must have 

 been due to something at the surface of the electrodes ; and 

 from the transient nature of the effect, I am inclined to ascribe 

 it to the temperature of the surface directly. 



Measurement of the Difference of Potential required to pass a 

 Spark through Air at different Temperatures, the Pressure 

 being constant. 



To investigate this question, I constructed the apparatus in 

 fig. 3. A glass cylinder c was fitted into two brass plates p 

 by means of grooves. The brass disks d, which were those used 

 in the previous experiments, were screwed on, the one to a 

 brass rod rising from the lower plate, the other to a rod 

 moving inside a tube fixed to the upper plate ; and they 

 were set at a distance of *9 centim. apart. The upper plate 

 contains an orifice to allow the air to escape when the 

 lower plate is heated, and a hole for the insertion of the 

 thermometer. The lower plate was put to earth, and the 



