OF ARTS AND SCIENCES. Ill 



one of 60.4 ; the height of the cyHnders was 92 cm. The capacity 

 was computed from the formula 



C = i— ; 

 log - 



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 bouiidiiry of the cylindrical plates was considered so 

 large in comparison with the distance between them that the boun- 

 dary 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 — & = distance between cylinders. 

 y8 = 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 cen- 

 tre of a large unoccupied room. The length of this circuit was about 

 fifty feet. It returned upon itself to the sparking terminal of the air 

 condenser. The jar was charged by a Iloltz 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 thou- 

 sand 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 ter- 

 minals of tin, between which the discharge took place. A concave 

 silvered glass mirror, of 313 cm. radius and 16.5 cm. aperture, placed 

 at a distance of 230 cm. from the spark, received the image of the 

 spark and reflected it back to the revolving mirror. From the revolv- 

 ing mirror the image was reflected to a photographic plate, which was 

 at a distance of 259.7 cm. from the rotating mirror. 



The adoption of a plane revolving mirror, and a stationary concave 

 mirror of long focus, enabled us to place the photographic plate at a 



