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LXXTI. On Electric Currents in Air at Atmospheric Pressure, 

 with Remarks on Induced Contact Electromotive Forces. By 

 Prof. A. Anderson, ALA., and H. N. Morrison, B.Sc* 



IN 'The Theory of Ionization of Gases by Collision,' p. 80, 

 Prof. Townsend seems to hold the opinion that it is 

 impossible for a current of electricity to pass through the 

 air space between two electrodes when the potential difference 

 is as low as 30 volts, the air being at atmospheric pressure, 

 even i£ the distance between the electrodes is less than the 

 wave-length of sodium light. In support of this opinion, he 

 refers to experiments made by Almy, who used spherical 

 electrodes of about *007 cm. in diameter. The area of an 

 electrode was, consequently, about the sixty-seventh part of 

 a square millimetre, and if, as we believe to be the case, 

 there are such currents, even for potentials much less than 

 30 volts, and for distances between the plates much greater 

 than the wave-length of sodium light, it is hardly to be 

 expected that they would be easily detected where such 

 small electrodes are used. It is probable that such currents 

 are due to the motion of electrons detached from the surface 

 of one of the metals, and set in motion by the electric force 

 between the electrodes; and, other things being the same, 

 they would be proportional to the area of the surface from 

 which the electrons are drawn. They cannot be explained 

 by the drawing together of the opposing plates by the 

 electrostatic force between them. In the apparatus used 

 by us the electrodes were two parallel circular copper 

 plates, the area of each being about 7 sq. cm. The upper 

 plate was connected with one pair of quadrants of an electro- 

 meter and the lower with the positive pole of a battery of 

 cells. Every precaution was taken to ensure insulation 

 where insulation was necessary, and to guard against any 

 effects other than that sought for. If the plates are at a 

 distance of, say, '005 cm. and the potential of the lower 

 plate be 30 volts, that of the upper plate being zero, the 

 force between the plates will be about the weight of one- 

 tenth of a gramme. No appreciable displacement of the 

 plates could be produced by such a small force, and yet, 

 under these conditions, there is a decided current. That 

 these currents pass through the air seems to be put beyond 

 doubt by another effect which takes place at the same time. 

 Let the lower plate, B, be connected with the positive pole 

 of the battery, and let the upper plate, A, which can be 

 moved by means of a micrometer-screw be earthed. After a 

 * Communicated by the Authors, 



