v1] PROPERTIES OF THE RADIATIONS 171 
Physical actions. 
112. Some electric effects. Radium rays have the same 
effect as ultra-violet light and Réntgen rays in increasing the 
facility with which a spark passes between electrodes. Elster and 
Geitel’ showed that if two electrodes were separated by a distance 
such that the spark just refused to pass, on bringing near a specimen 
of radium the spark at once passes. This effect 1s best shown with 
short sparks from a small induction coil. The Curies have ob- 
served that radium completely enveloped by a lead screen 1 cm. 
thick produces a similar action. The effect im that case is due to 
the y rays alone. This action of the rays can be very simply 
illustrated by connecting two spark-gaps with the induction coil in 
parallel. The spark-gap of one circuit is adjusted so that the 
discharge just refuses to pass across it, but passes by the other. 
When some radium is brought near the silent spark-gap, the spark 
at once passes and ceases in the other. 
Hemptinne? found that the electrodeless discharge in a vacuum 
tube began at a higher pressure when a strong preparation of 
radium was brought near the tube. In one experiment the dis- 
charge without the rays began at 51 mms. but with the radium 
rays at 68 mms. The colour of the discharge was also altered. 
Himstedt* found that the resistance of selenium was diminished 
by the action of radium rays in the same way as by ordinary light. 
F. Henning‘ examined the electrical resistance of a barium 
chloride solution containing radium of activity 1000, but could 
observe no appreciable difference between it and a similar pure 
solution of barium chloride. This experiment shows that the 
action of the rays from the radium does not produce any appreciable 
change in the conductivity of the barium solution. The amount 
of radium present was too small to obtain the relative conductivity 
of the radium and barium solution. 
Specimens of strongly active material have been employed to 
obtain the potential at any point of the atmosphere. The ionization 
due to the active substance is so intense that the body to which it 
1 Wied. Annal. 69, p. 673, 1899. 2 C. R. 133, p. 934, 1901. 
3 Phys. Zeit. p. 476, 1900. 4 Wied. Annal. p. 562, 1902. 
