114 PROCEEDINGS OP THE AMERICAN ACADEMY 



ployraent of the formulas for geometric mean distance * for two lines 

 whose directions intersect at right angles. 



Lord Rayleigh has given the following formula for inductance under 

 rapid oscillations : — 



L' = I (a + i/^) ; 



\ y 2pi/ 



in which ^ is a constant, depending on form of circuit ; /a is per- 



2 TT 



meability ; R is resistance ; p = — , where t is time of oscillation ; 



I is length to and fro of inductance circuit. 



The final value of Z' for our case is L' = 39697. 

 The radius of the wire employed was a =■ .0501 cm. 

 The length was measured in three sections : — 



No. 1, length 1197.0 cm. distance between wires \ = 31.55 cm. 

 No. 2, « 281.0 " " " b^ = \Q.lcm. 



No. 3, " 103.0 « " " ^>3 = 11.3cm. 



The ohmic resistance of the wires was .742 x 10® for direct current, 

 and 1.54 X 10® for alternating currents of period t = ,0000031. 



The difficulty in the process of photographing the spark consisted 

 in discharging the air condenser through the induction circuit at the 

 instant the revolving mirror was in a position to reflect the image of 

 the spark to the photographic plate. The terminal connected with 

 the revolving mirror, which allowed the electrical discharge to pass 

 when the mirror was in the desired position, had to be adjusted with 

 extreme care. The speed of the image at the photographic plate was 

 about one mile per second. 



The photographs were measured by means of a dividing engine. 

 At first an objective of low power was used on the microscope of the 

 dividing engine. It was found, however, that a simple cross hair, un- 

 aided by a lens, moving directly against the negative, was better than 

 any eyepiece. Measurements were made of the intervals between 

 the electrical oscillations at both terminals. 



In later experiments a smaller air condenser was employed, for 

 reasons which will appear in the conclusions of our paper. 



A summary of the details and dimensions employed is given 

 herewith. 



Small Air Condenser (No. 2), cylindrical. 

 19 zinc cylinders. 

 Height, 30.47 cm. 



Diameter of inner cylinder, 7.60 cm. 

 Diameter of outer cylinder, 25.95 cm, 



* Maxwell, § 692, Vol. II. 



