Manometric Observations 'at Poles of the Elective Arc. 957 



between the two ionic streams. This may be seen by con- 

 sidering the magnitudes of the effects produced by each sign 

 separately. Taking Chattock's expression for electric wind- 

 pressure, a current density of 10 amperes in an arc of 1 cm. 

 length carried entirely by electrons of, let us say, mobility 

 10,000 cm./sec./volt/cm. would give rise to a difference of 

 pressure between the electrodes of about 10 5 dynes per 

 sq. cm., i. e. about 100 cm. of water. The corresponding 

 calculation for the same current carried entirely by the 

 relatively slow-moving positive ions gives a pressure 

 difference of several atmospheres. 



Now the results show that in the main body of the arc 

 the gradient of pressure, if any, is less than 1 dyne per cm. 

 of arc. The mobilities of the ions in an arc are not known. 

 But if it is assumed that the negative ions are electrons, 

 the ratio of the mobilities is at least 500. Accepting this 

 value for the time being, this means that in this region 

 only 1/5 per cent, of the current is carried by positive 

 ions *. Thus, on this view, though the presence of this 

 positive ionic stream is essential to bring about the observed 

 balance, the main outcome of this result is that for most 

 practical purposes we may regard the whole of the current 

 as being carried by electrons. 



It should be pointed out, however, that the above num- 

 bers must be modified if any appreciable part of the current 

 is carried (1) by negative ions of molecular magnitude 

 (see the following paper by one of us, A. M. T.), or (2) by 

 ions not moving viscously or, to give the specific case pos- 

 tulated by Daffield, Burnham, and Davies, by electrons 

 projected from the cathode and travelling with a speed 

 large compared with that which they would acquire in the 

 field itself. In the latter case the estimated fraction of 

 current carried by positives would be still less. But this 

 matter is referred to again below. 



Experiments. 



The first experiments were made with a horizontal straight 

 arc of soft carbons 1*3 cm. diameter, through each of which 

 a hole 3*5 mm. diameter had been bored by removing the 

 soft core. All experiments were carried out at atmospheric 

 pressure. The pressure at either anode or cathode was 



* These experiments are briefly mentioned in a note in the Phil, Mag. 

 (Dec. 1920). Since that time it has been shown that the residual wind- 

 pressure therein referred to originates near the electrode and not 

 throughout the body of the arc. 



