CONTEMPORARY ADVANCES IN PHYSICS 111 



proceeded to build an apparatus in which the maximum voltage should 

 not go above a quarter of a million, but in recompense the stream of 

 protons should be raised to a hundred microamperes. Another 

 alteration: previously the stream had been a mixture of protons with 

 heavier ions and neutral particles — now Oliphant and Rutherford 

 introduced a magnetic field, adjustable and strong enough to bring 

 either the protons or the more massive ions separately against the 

 target. The magnetic field also assures that all the particles striking 

 the target shall have nearly the same speed, something not completely 

 guaranteed by the constancy of the voltage. 



The scheme of this device is sketched in Fig. 2, where the course 

 of the proton-stream is traced (rather too pictorially, I fear!) in a 

 sweeping arc from its origin in the discharge-tube R, to the target T 

 where the element to be transmuted awaits the impacts. In the 

 discharge-tube all the parts are of steel, and the block C and cylinder 

 B conjointly form the cathode, while the oil-cooled block D and 

 cylinder A conjointly form the anode. This unusual material and 

 structure are required partly to minimize cathode-sputtering, and 

 partly to take care of the great amount of heat which is steadily 

 developed in the tube, inasmuch as for the best supply of protons a 

 voltage of 20,000 and a current of many milliamperes are demanded. 

 Something like a twentieth of the current in the discharge is borne 

 through the hole in the cathode by protons (and other positive ions 

 of greater mass, if such there be) ; and in the space between C and E 

 these particles receive from an electric field most of the kinetic energy 

 with which they strike the target. In this space and in the region 

 where the magnetic field comes into play, the density of the gas must 

 be kept extremely low, despite the fact that there is an open passage 

 into these spaces from the discharge-tube where the density must 

 always be great enough to sustain the discharge and the supply of 

 protons. This is a task for powerful pumps, which must be kept 

 continuously at work pumping away from the lower chambers the gas 

 which is steadily draining out of the discharge-tube through the hole 

 and must as steadily be replenished by feeding fresh hydrogen in from 

 above. It is no small part of the difficulty of the experiment, that 

 the discharge-tube and the source of its power and the source of its 

 hydrogen must all be maintained at scores or hundreds of thousands 

 of volts above the potential of the ground, in order that the observing- 

 apparatus may itself be at ground-potential. The transmutations are 

 observed by detecting the fragments which issue through the very 

 thin mica pane of the window W. 



