284 CARNEGIE INSTITUTION OF WASHINGTON. 



divisions per volt has been obtained by this method, and such a sensitivity is 

 sufficient to result in an appreciable kick in the fiber each time an a particle 

 is shot out in the ionization chamber. 



A high potential is required on the wall of the ionization chamber, so that 

 the ions may be brought to the central rod sufficiently rapidly to cause a sharp 

 kick. This potential must be very constant, for if the electroscope is sensitive 

 to 1,000 divisions per volt, it would respond to the extent of about 0.1 division 

 if the battery on the ionization chamber fluctuated in potential by 10~* 

 volt. Hence, if the battery in question had an e. m. f. of 500 volts, it would 

 have to be free from fluctuations to the extent of one part in 5,000,000. It 

 would, in general, be impossible to obtain such a degree of constancy, but the 

 equivalent of obtaining it has been secured as follows : To the system connected 

 to the fiber is attached a brass cylinder, surrounded by another brass cylinder, 

 which, however, does not touch it. Although the volume of the attachment is 

 small, the capacity of the portion inclosed by the hollow cyUnder is comparable 

 with, and may be made nearly equal to, the capacity of the portion of the rod 

 surrounded by the large vessel. The two ends of a megohm are connected 

 respectively to the large vessel and to the outer cylinder of the attachment, 

 the mid-pomt of the megohm being connected to the case of the electroscope. 

 A battery of 1 ,000 volts is also connected to the two ends of the megohm. It 

 will be obvious that, under these conditions, and when the capacities above 

 referred to are adjusted to equality, fluctuations of the battery potential maj'' 

 take place without affecting the potential of the insulated system. 



With the above apparatus it has been possible to detect and photographi- 

 cally record the ionization by a single a particle. 



Experiments have also been made on a new form for plates of the unifilar 

 electrometer, with a view to increasing still further the range and sensitivity 

 of the instrument. 



