X-ray Electrons. 1111) 



coil and mercury break, with an auxiliary mechanical 

 rectifier. A maximum input of 1000 watts was used. 



The magnetic field was provided by two large air-core 

 coils (H t ) placed such a distance apart as to produce a 

 sensibly uniform field over the area of the camera. Auxiliary 

 small coils H 2 in series with Hj were so arranged as to 

 compensate the external field of Hj over the path traversed 

 by the cathode rays between C and T. 



The uniformity and value of the field was investigated bv 

 search coil and ballistic galvanometer in the usual way. A 

 current of 7 amperes in the coils gave a uniform magnetic 

 field of 101-1 Gauss*. 



It is clearly an advantage in this kind of work to use air- 

 rather than iron-cored wires, since 



(1) The value of the field is always directly proportional 



to the current traversing the coils, audit is therefore 

 easy to adjust the fie;d to any required value with- 

 out troublesome remeasurement. 



(2) A small accidental variation of current produces 



only a proportionate change in the field. 



(3) The compensating coil having been set lor any one 



value of the field is set also for any other value. 



The disadvantage is that rather large electrical energy is 

 consumed bv the field coils— in the present case, about 

 500 watts. 



The radii of curvature of the electrons varied from about 

 3 to 6 cms. 



Taken as a whole, the apparatus just described is very 

 troublesome to manage owing to the difficulty of keeping 

 the numerous metal, quartz-glass joints air-tight. In spite 

 of water-cooling, the joints too frequently give way. Its 

 main recommendation is on the ground of economy, since 

 the expensive Coolidge tube and alternator transformer is 

 not required on account of the high efficiency of the 

 arrangement. 



This efficiency is obtained 



(1) Because the X-ray electrons are liberated from three 



surfaces within J. 



(2) Because the target T being very near J, the intensity 

 m of the X-rays is very great. 



The second reason just given is particularly important. An 

 ordinary X-rav bulb could not easily be arranged nearer than 

 say 10 cms. to J, with a consequent 100-fold loss of intensity. 



* Since H 2 is small compared with i\ l , one would expect its external 

 field to be negligible in H r This was proved by showing that the value 

 of RH was constant for some definite speed group of electrons and thus 

 independent of the actual path traversed in H x . 



