42 Prof. E. Wiedemann on the 



trophorie machine the simple introduction of an air-spark is 

 sufficient. 



2. Phenomena at different Distances of the Electrodes. — It is 

 of importance to be able to trace the phenomena which result 

 when the distance of the kathode from the anode is changed 

 within somewhat wide limits. It is essential that the vessel 

 containing the electrodes should not be opened, for with each 

 such opening the pressure would be altered : and the pressure 

 under which the first determinations were made is, especially 

 when low pressures are employed, not to be obtained again. 



In order to carry out experiments in the direction indicated, 

 I have constructed the following apparatus (Plate II. fig. 1) : — 



A is the discharge- tube, which communicates with the air- 

 pump at e ; a is an electrode firmly melted in, which is 

 connected with the machine in the manner already described *. 

 A second electrode, insulated in the usual way, is inserted at 8 ; 

 the lower part has melted on to it the outer portion (e) of a 

 ground glass joint. The inner portion (d) of the same forms 

 one end of a U-tube, B 0, whose longer limb B is about 900 

 millim. long, and its shorter limb C about 500 millim.; at the 

 bend it is provided with a tap, and a platinum wire is melted 

 in at E. The two portions of the joint are connected by caout- 

 chouc bands at s s f and 1 1' in the manner already described |- 

 So much mercury is next introduced into the U-tube that it 

 stands in each limb about 400 millim. high. 



The second electrode is formed by the plate /3, which can 

 be replaced by a point &c. It is fastened and connected with 

 the machine as follows : — A thin platinum wire is melted into 

 one end of a thin glass tube of suitable length, and the end 

 within the glass tube covered with a little mercury. Into this 

 dips the end of an aluminium wire which traverses the whole 

 length of the tube, and on the top of which the plate is screwed. 

 By this arrangement the whole conducting wire up to the 

 electrode was covered with glass. In other cases the metallic 

 wire was left uncovered. 



The rod carrying the electrode thus floated with its lower 

 end in the mercury in the limb B. By pouring mercury into 

 C, or allowing it to run out from A, the level could be raised 

 or lowered, and thus the electrode (S made to approach or 

 recede from the plate a. To regulate the motion it was neces- 

 sary to provide the rod with some guidance : the simplest plan 

 was apparently to make the tube B so narrow that the rod can 

 just move up and down it ; there was, however, so powerful a 

 capillary action, that not only in this case, but also when much 



* Wied. Ann. x. p. 206 (1880) ; Phil. Mag. vol. x. p. 360. 

 t Ibid. p. 209 (1880) ; Phil. Mag. vol. x. p. 362. 



