Vacuum Gold-leaf Electroscopes in Rarefied Gases. 719 



side through the glass cover of the electroscope, are placed 

 on the two sides of the gold leaves ; and the leaves are long 

 enough to touch these plates when their divergence reaches a 

 certain amount. As the /3-rays of the radium emanation 

 escape the a-ravs electrify the gold leaves, which then diverge ; 

 and, diverging, they come in contact with the side-plates. 

 They are then discharged, and fall together, but again 

 become electrified as before. A periodic action thus goes 

 on, the gold leaves alternately diverging and collapsing. 

 In the radium clock which we have constructed, and with 

 a vacuum of 2 V ^«*j the period of the gold leaves is about 

 3^ minutes. 



While observing the radium clock, and endeavouring to 

 improve the vacuum of the apparatus, a Bunsen burner which 

 was being used was placed in rather close proximity to the 

 clock. It had a long powerful flame, and it was observed, after 

 a short time, that the gold leaves of the clock were apparently 

 repelled by the flame. 



It is impossible to make the gold leaves so symmetrical that 

 they shall both strike their respective fixed side-plates at the 

 same instant. One must strike first, and then both are 

 discharged. In our case, that which we call the " left-hand 

 leaf," looking at the apparatus from what we will call the 

 " front," had been striking first. The Bunsen burner was, 

 by chance, standing on the left. It was noticed that the leaves 

 did not hang symmetrically, as shown in fig.l, but were repelled 

 and hung as shown in fig. 2 (p. 750) ; and as their divergence 

 increased the " right-hand " leaf was the first to strike on its 

 side-plate, and it was found at the same time that the period 

 of the clock was greatly increased. 



Experiment 2. — A common soldering-bolt was next heated, 

 sometimes to red heat, or almost to white heat, and, later on, 

 to temperatures far below red heat but higher than that of the 

 surrounding atmosphere ; and it was found that, when this 

 was brought near, the leaves of the radium clock behaved 

 much as the}' did in the presence of the flame. The same 

 effects were obtained when a spirit-flame, or a candle-flame, 

 or the light from a Nernst lamp, was used instead of a Bunsen 

 burner. 



Experiments. — Even the heat of the hand was found to be 

 quite sufficient to cause repulsion of the gold leaves of the 

 clock. By bringing up the warm hand close to the outside 

 cover of the clock the leaves could be driven slowly, first in 

 one direction and then in the other ; and they could in this 

 way be prevented from striking on the side -plates ; thus the 

 * M denotes one-millionth of standard atmospheric pressure. 



