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CIII. On the Forces acting upon the Poles of the Electric Arc. 

 By Professor A. M. Tyndall*. 



IN the preceding paper by Mr. H. E. G. Beer and the author 

 it is shown that the hydrostatic pressure at the poles of 

 the electric arc, first observed by Dewar, may be regarded as 

 arising from an electric wind set up by the motion of space 

 charges in the vicinity of the electrodes ; and, further, that as 

 the main body of the arc contributes practically nothing to 

 the wind, it must contain practically equal quantities of each 

 sign of ion per cubic centimetre at all points. 



Reference is also made to the work of Duffield, Burnham, 

 and Davis, who observed the existence of a mechanical 

 "pressure on each pole of the arc tending to thrust them 

 apart. In looking for the cause of this pressure, Duffield 

 rules out all motion of ions which arises from the electro- 

 static forces within the arc ; and adopting Pollock's view of 

 the projection of high-speed electrons from the cathode, he 

 regards the pressure on the cathode as representing the recoil 

 of this projection, while that on the anode is due to the trans- 

 ference of the momentum of the electrons to it either directly 

 by the electrons themselves or indirectly by its communication 

 to the gas as drag. 



It seems to the author it is not legitimate to rule out 

 entirely the effects of the electrostatic forces within the arc, 

 and that it is desirable to re-examine the theoretical basis 

 of the Duffield effect in the light of the further information 

 which the study of the Dewar hydrostatic pressure has 

 provided. 



When an ion moves viscously anywhere in a gas between 

 two electrodes, it receives momentum from the electric field 

 and simultaneously hands it on to the gas, while the electrodes 

 experience reaction equal and opposite to the force acting on 

 the ion. 



If the electrodes are so large and so near one another that 

 there is no appreciable loss of momentum from the gas 

 between them to that outside, the momentum in the gas will 

 be passed back to the electrodes and will be exactly equal 

 and opposite to that which they receive from the electrical 

 reaction. This is, however, only general when the forces on 

 both electrodes are taken together. 



Before a similar general statement is possible for the 

 electrodes taken separately, one special condition must 

 obtain — namely that of symmetry. 



* Communicated bv the Author. 



