122 



W. Duane — Effect of a Magnetic Field 



maintains one of the plates at a constant potential, and a sensi- 

 tive electroscope connected with the other serves to measure 

 the ionization current between them. An air-tight box sur- 

 rounds the plate, and by reducing the gas pressure in it the 

 length of the mean free path of the ions can be increased. The 

 box is suspended between the poles of an electromagnet in such 

 a way that the lines of magnetic force are parallel to the metal 

 plates and perpendicular to the beam of rays. Under these 

 conditions an ion, after it has been formed, moves under the 

 action of electric and magnetic forces that are at right angles 

 to each other, and instead of following the line of electric 

 force it is deflected off to one side by the magnetic field. If 



Fig. 1. 



To battery 



To earth 



the length of the free path is long enough, and the magnetic 

 force large enough, the ion should be deflected so far to one 

 side that it does not reach the plate serving as electrode. Thus 

 under proper conditions the magnetic field should diminish 

 the ionization current measured by the electroscope. 



The magnetic field, of course, deflects the /3-rays and the 

 secondary rays, and thus diminishes the current ; but the ioni- 

 zation due to these rays is small compared with that due to the 

 a-rays (which are not suppressed by the magnetic force), and 

 it will be seen from the experiments that the decrease in the 

 current due to the stopping of the fi- and secondary rays is 

 small and easily distinguished from that due to the deflection 

 of the ions. 



Figure 1 represents the arrangement of the apparatus. A 

 tiny glass bulb A containing radium emanation (sometimes as 

 much as that in equilibrium with 0'2 gram of radium) serves 

 as the source of the ionizing rays. The glass bulb is supported 

 close to a small mica window, l mm in diameter, and both the 

 glass and the mica are so thin (their combined power of absorp- 

 tion not exceeding that of 3 cm of air at atmospheric pressure) 



