254 ME. E. M. WELLISCH ON THE MOBILITIES OF THE 



position of K' with regard to K could thus be read off at once ; but when greater 

 accuracy was required the position of K ; was ascertained by means of a cathetometer. 

 C is a capacity of about 7 microfarads inserted in parallel with the primary of the 

 induction coil in order to prevent sparking at the contact K'. When the spark was 

 entirely eliminated it was found that, provided the current as measured by the 

 ammeter Am was kept constant, the intensity of the Rontgen-ray flash was sensibly 

 constant at each discharge. 



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Fig. 4. 



AB and A'B' denote the ionisation chambers which are described below ; the upper 

 electrodes A and A' could, by means of the key Y, be connected separately to the 

 insulated pair of quadrants of an electrometer, or could be connected together either 

 with or without connection with the electrometer. The lower plate B' of the chamber 

 A'B' is connected with the point b' of the key Z, and from the diagram it can be seen 

 that, when I/ and c are joined, B' is permanently earthed, while, when 6' and p are 

 joined, B' can be put permanently either at a positive or negative potential by means 

 of the key k. The lower plate B of the chamber AB is connected with the point 6 

 of the key Z ; thus, when b and e are joined, B is permanently earthed, while, when 

 b and m are joined, B is at the potential of the movable arm D of a magnetic 

 relay M. When this arm is in contact with the point P, B is thus at zero potential, 

 while, when the arm is in contact with Q, B is at the same potential as the point ft, 

 a potential which, as above described, is reversed in sign by breaking the contact at 

 K. The object of this relay is described below under section 4. 



The potentials necessary to establish the electric fields in the ionisation chambers 



