[ 216 ] 



XX. On Certain Types of Electric Discharge. By Prof. 

 D. JSL Mallik, F.R.S.E., and Prof. A. B. J3as, M.Sc* 



1. TN previous papers (Phil. Mag. Oct. 1908, Oct. 1912, 

 _1_ July 1916) we studied the behaviour of electric 



discharge in a De La Rive tube, under gradually decreasing, 

 pressure. In the present series of experiments, tubes of 

 ordinary pattern (with electrodes consisting of thin rods) 

 were used, in order to verify the various theoretical deduc- 

 tions, previously arrived at. 



2. Three tubes (^ = 344 cm. in length, T 2 = 14*5 cm., 

 and T 3 = 4*2 cm.) were used in parallel, in order that 

 the effects of length and decreased pressure on the 

 character of the discharge should be clearly observable. 

 The induction-coil used gave a spark-length of 19*94 cm. in 

 air, with a spherical electrode of 3 cm. in diameter. 



3. At a pressure of 73 mm. no discharge passes in T l5 it is 

 spindle-shaped in T 2 while in T 3 it is in the form of a band. 

 As the pressure is further reduced at 20*5 mm. the discharge 

 in T\ is spindle-shaped, T 2 shows a band which was the 

 stage reached in T 3 at the higher pressure, while in T 3 the 

 band is changing into a glow discharge. 



4. On the theory previously worked out, the spindle-shape 

 is due to the mutual repnlsion between the various streams 

 of discharge, which is seen to be operative, in spite of the 

 symmetry of the electrodes. 



5. Moreover, during the band stage, according to theory 

 previously worked out, the result of ionization is annulled 



^£ 

 by recombination. ^Now ionization depends on — , where X 



is the electric intensity along the discharge and p the 



pressure. This quantity, therefore, depends on j- where I is 



the length of the tube. The effect of decreasing the length 

 must, therefore, correspond to that of increasing the pres- 

 sure. As, however, the potential difference itself depends on 

 length and pressure (arts. 15 and 16), there is not as yet 

 sufficient material for working out the law of this corre- 

 spondence. 



6. It is found that it is only when the discharge is in the 

 form of a thin band or a single stream of discharge, that 

 there is an action (attraction or repulsion) when the discharge- 

 tube is placed in a transverse magnetic field. In the actual 

 -experiment, a rectangular coil of wire, carrying current, two 



* Communicated by the Authors. 



