Transformation of Thorium and Actinium Emanation. 723 



electrode leading into the ionization-chamber was joined, were 

 controlled by small electromagnets M^ and M 2 . The latter 

 were energized at will by several storage-cells in series with 

 a resistance R, a three-pole double-throw switch S,. and a 

 massive half-seconds pendulum P provided with a needle- 

 point dipping into a mercury cup. When both keys K x and 

 K^ were down, the condensers, gold leaf, cup C, and electrode 

 were eartbed. With tbe switch S thrown into one position, 

 the earthing key K x was raised by the next contact of the 

 pendulum and the gold leaf allowed to charge : in the other 

 position of the switch the key K 2 was raised, disconnecting 

 the ionization-chamber from the gold leaf, thus permitting 

 the deflexion of the gold leaf to be accurately determined. 

 Small catches carried on upright brass strips supported the 

 keys after being raised, and could be released at will by cords 

 attached to them. The plate of the electroscope was joined 

 through a water resistance and a potentiometer to a battery 

 of 200 volts. By moving the arm of the potentiometer the 

 sensibility of the electroscope could be adjusted. It varied 

 in different experiments from 20 to 40 divisions per volt. 

 Thick lead plates protected the chamber B, cup C, capacities 

 and electroscope from the direct radiation of the active 

 material used as source. A small sounder, placed in series 

 with the pendulum circuit, rendered it easy to note the time 

 at which the keys K x and K 2 were raised. 



Order of Experiment. 



The natural leaks with the various capacities in parallel 

 with the electroscope were determined at the beginning of an 

 experiment. In the case of actinium emanation the order of 

 the experiment was as follows : — The capacity marked "1 m.f. 

 was connected in parallel with the leaf, both keys K : and K 2 

 being earthed. The pressure in the chamber B was lowered 

 usually about 2 or 3 cm., as recorded on the manometer, and 

 the stopcock leading to the pump closed. The pendulum 

 was set vibrating through a small arc and the counting of 

 these vibrations begun, when the stopcock leading to the 

 tube A, containing the emanation, was opened. This was 

 closed after 3 or 4 seconds. Since the sounder and keys Kj 

 and K 2 were controlled by the beats of the pendulum, it was 

 possible to record the exact time at which these keys were 

 raised. When K : was raised the deflexion of the gold leaf 

 began, and after 2 seconds, i.e., 4 beats, the key K 2 was 

 raised, thus disconnecting the chamber from the electroscope. 

 The keys were then allowed to fall, and the total deflexion 

 recorded. As decay proceeded, smaller and smaller capacities 





