184 Mr. T. Harris on Distribution of Electric Force 



the cathode c, through the earthed tube anode a, and the 

 earthed metal tube t, entering the discharge-tube through 

 the hole, h, bored in the side of the tube, emerging from 

 the discharge-tube through a similar hole diametrically 

 opposite, whence it proceeded through the earthed metal 

 tube t' to the wiilemite screen, W. Thus the cathode rays 

 were shielded from electrostatic disturbances by earthed 

 metal tubes during the whole of their journey, except when 

 passing through the field it was required to measure. 



The main discharge M was run from a six-plate Wimshurst, 

 the secondary discharge S from a small Wimshurst, both 

 machines motor-driven with both positive poles earthed. 

 The voltage across M was measured by a Braun electroscope, 

 that across S by measuring the equivalent spark-gap G. 



The application of the method for the measurement of big 

 forces is difficult. It is necessary to use two discharges, one 

 running at a very much greater voltage than the other. 

 If the gas is at the same pressure in both cases this can be 

 accomplished by putting one cathode in a constricted space, 

 so that the charged walls surrounding it require the use of a 

 big voltage to cause a discharge. This necessitates the use 

 of a small cathode placed in a narrow-bore tube, and the 

 difficulty arose that the discharge was inclined to be very 

 irregular for P.D. greater than 15,000 volts, e. q. taking- 

 place from the edges of the cathode instead of forming a 

 steady beam of rays from the central portion. So much 

 time was spent in surmounting this difficulty that a full 

 description of the design of the tube S is not unwarranted. 



The cathode is shown in fig. 2. It was constructed as 



Fig. 2. 



Capillary tubing 



, //,'/■'////■ '///,' '/"//■ ". : , _ ~\-- Aluminium c< 



fusible metal 



Cathode 



follows. A small bulb was blown in a piece of capillary 

 tubing and drawn out until it was of the same external 

 diameter as the rest of the tube. This was cut through and 

 ground down until the plane of the end was perpendicular 

 to the axis of the tube. A piece of aluminium rod which 

 fitted the capillary was pushed through until it projected a 

 little past the end. The space between this and the walls 

 was filled with pieces of fusible metal, which on warming 

 fused and on cooling solidified and expanded, completely 

 filling the whole of the space between the rod and the glass. 

 The metal end was ground down flush with the glass and 



