188 Mr Almy, On a Diminution of the Potential Differmee, etc. 



charge changes, the potential difference increases with increasing 

 cathode area. And it is only with this second sort of discharge 

 that the effect of the magnetic field is marked, or uniformly 

 consistent. 



The transition from the one type of discharge to the other 

 is very largely dependent upon external conditions — in fact it 

 may be entirely prevented. If, for instance, the discharge tube 

 outside the cathode and clear up to the anode be coated with 

 tinfoil, connected to earth (the cathode being also earthed) the 

 character of the discharge remains the same independent of the 

 cathode area. In this case the discharge takes place entirely 

 by stream lines, — usually by a single stream — which pass along 

 the side of the tube, accompanied by luminous Hashes, often with 

 crackling noise as a spark discharge. 



The action of the magnetic field producing a diminution of 

 potential difference between the electrodes is inseparably con- 

 nected with the transition to the stream -like discharge, the 

 action of the magnet is to always produce that type of discharge. 

 Furthermore, the tinfoil coating of the tube entirely eliminates the 

 effect of the magnetic field, and there is a diminution of the P.-D. 

 between the electrodes due to the tinfoil, of the same magnitude 



as that produced by the magnet. 



The suggestion offers itself that the 

 effect of enlarging the cathode is to in- 

 crease the charge which collects on the 

 walls of the tube, and this charge acts as 

 a counter E.M.F. to increase the P.-D. ne- 

 cessary for discharge in the tube. 



To test this idea two different experi- 

 ments were tried. 



A tube, as shown in Fig. 2, was 

 constructed. The diameter of the smaller 

 tube was about 15 mm., that of the large 

 part being about (50 mm. Aluminium 

 cathodes of 1 cm. area were at each end, 

 and a ring of aluminium wire at the centre 

 was the anode. 



The effect of a magnetic field, trans- 

 verse and parallel, to the direction of the 

 discharge was observed with each end of 



k 



0? 



f 



l <p r^- 



the tube ; then the effect of surrounding 



j$ the tube with tinfoil connected to earth, 



Fio. 2. was tried. 



These results seem to indicate that 

 the effect produced by the magnetic field is not due to the 

 removal of a static field, for at the different ends of the tube 



