ON THE DISCHAEGE OF ELECTRICITY FKOM POINTS. 73 



respect. It was hoped that the results thus obtained would have been 

 sufficiently complete to submit to the meeting this year, but owing to 

 unavoidable delays in carrying out the experiments this is not the case. 

 The main conclusion, however, appears, so far, to be that in the formation 

 of sparks the anode is a far more important factor than the cathode, 

 disturbing influences having little or no effect when applied to the latter, 

 though they may prevent the passage of sparks altogether if applied to 

 the anode. This is in accordance with Lehmann's recently described 

 experiment on the effect of a strong blast of air upon sparks. It is also 

 borne out by experiments now in progress on the critical distance 

 between a point and a plate at which discharge from the point changes 

 from the form of a silent glow to that of a spark, the distance being 

 appreciably greater when the point is the anode than when it is the 

 cathode. 



Another line of investigation, which was indicated in last year's 

 report, has been followed up in the attempt to measure the ratio of the 

 mass of the gas molecules concerned in carrying a current of electricity 

 to the quantity of electricity carried. The discharge being arranged to 

 take place convectively along a non-conducting tube of uniform bore, the 

 slopes of mechanical pressure and electrical potential along its axis are 

 determined. The apparatus for this purpose has undergone many 

 changes, chiefly on account of the difficulty experienced in bringing the 

 electrometer to the potential of that part of the tube with which it is 

 connected. Stationary conductors within the tube were found to be 

 unreliable for this purpose, and a modification of the well-known ' water- 

 dropper ' has therefore been adopted in the latest form of the instrument. 

 The slope of potential is measured by observing the differences of 

 potential between successive pairs of positions one centimetre distant 

 from each other along the tube. The electrometer has thus to measure 

 small differences of potential at a high mean potential with respect to the 

 earth. For this purpose an instrument has been specially designed, from 

 which the leakage to earth seems negligible. To obtain the slope of 

 pressure in the discharge tube the motion of an indiarubber diaphragm, 

 separating two air-tight chambers, is observed, the chambei's being 

 connected, like the terminals of the electrometer, to successive pairs of 

 points along the tube, and the movement of the diaphragm being 

 magnified optically. A difference of pressure of about five dynes per 

 squai-e centimetre causes a movement of one scale division in the present 

 insti'ument, and this is found to be quite sufficient for the purpose. 

 With this apparatus it is hoped that reliable results will shortly be 

 obtained. 



Finally, preparations are being made to extend the measurements 

 already obtained of the strength of field at a discharging point to higher 

 than atmospheric pressures. For this purpose a powerful compression 

 pump and a strong discharge chamber are being constructed. Upon the 

 assumption that discharge occurs by the breaking down of electrically 

 polarised molecular chains in the gas, it is possible to obtain from such 

 measurements another rough estimate of the value of the electrical charge 

 associated with a gas atom, by extrapolating to the pressure at which 

 the gas molecules are brought close together. It seems desirable to 

 obtain all the evidence possible which bears upon this important point. 



Tour Committee ask for reappointment withont a grant. 



