Electrical Oscillations in Air. 325 



employ an air condenser instead of one of glass, in order to 

 detect, if possible, the effect of the medium of the dielectric 

 upon electrical oscillations. In order to obtain sufficient 

 capacity for a suitable spark, we were obliged to use the 

 cylindrical form of condenser. The first condenser we em- 

 ployed was made of sheet zinc and consisted of nineteen coaxial 

 cylinders. The inner cylinder had a diameter of 15*1 centim., 

 and the outer one of 60*4 ; the height of the cylinders was 

 92 centim. The capacity was computed from the formula 



in which I is the height, and b and a are radii. 



A correction for the ends was made as follows. The radius 

 of curvature of the boundary of the cylindrical plates was 

 considered so large in comparison with the distance between 

 them that the boundary was treated approximately as a straight 

 line. We may consider that each zinc cylinder constituted a 

 plate between infinite imaginary planes which were at zero 

 potential, these planes being equipotential surfaces. The 

 zinc cylinder was supposed to have its area extended by a 

 strip of uniform breadth around its boundary, and the surface- 

 density was assumed to be the same on the extended plate as 

 on the parts not near the boundary. Following Maxwell 

 (vol. i. § 196), we have 



— log 2 cos -rr for the correction for length. 



B = a — b = distance between cylinders. 

 {3 = thickness of cylinder. 

 I = height of cylinder. 



This air condenser was connected with a circuit of parallel 

 wires, which was carefully strung by means of silk thread 

 through the centre of a large unoccupied room. The length 

 of this circuit w r as about fifty feet. It returned upon itself 

 to the sparking terminal of the air condenser. The jar was 

 charged by a Holtz machine, which worked fairly well under 

 all conditions of the atmosphere. The revolving mirror was 

 a plane one, 4x5 inches, silvered upon the front face. It 

 revolved upon a horizontal axis with an average speed of 

 three thousand revolutions per minute. The frame which 

 carried the mirror bore also a brass arm provided with a 

 minute brush, which rubbed upon a brass sector let into a 

 large disk of ebonite. When the brush rested upon this brass 

 disk the electrical charge could pass to two terminals of tin, 



