CONDUCTION THROUGH GASES 135 



An electric machine capable of yielding sparks 

 was invented manyyears ago during the eighteenth 

 century; and the question soon arose whether such 

 sparks were of the same nature as the lightning 

 flash — whether the roll of the thunder was but 

 the reiterated crackle of the stupendous electric 

 machine of the atmosphere, echoing amid the con- 

 volutions of theclouds. The questionwas answered 

 in the year 1752 by Franklin, who floated a kite 

 in the air, and, when the string was made a con- 

 ductor by a shower of rain, was able to draw the 

 confirming sparks from its lower end. 



A very great electric force is required to main- 

 tain a visible discharge through a few centimetres 

 of air at the atmospheric pressure, and the initial 

 force needed to start the process is still larger. It 

 was soon found, however, that a reduction of 

 pressure facilitated the passage of the spark, and 

 that it was much easier to send the discharge 

 throuofh a vessel from which the air had been 

 partially exhausted by means of an air-pump. To 

 illustrate this, platinum wires, to act as electrodes, 

 are sealed into little glass tubes containing air at 

 low pressure. For many years these vacuum 

 tubes, as they are called, were the electrical play- 

 things of the laboratory and popular lecture-room. 

 Recent discoveries have raised them from the 

 position of scientific toys to the rank of pieces of 

 apparatus, whereby have been made some of the 

 greatest discoveries in physical knowledge that 

 the present generation has seen. 



Through such a tube, in which the pressure of 

 the air is only a small part of an atmosphere, a 

 discharge may readily be passed by the aid of a 

 voltaic battery and an induction coil, or by the use 



