the Velocity of Electric Waves, 215 



the wave was slightly altered for a metre on each side of J, 

 and the bolometer showed a slight accumulation in the 

 immediate neighbourhood of the spark-gap. This was probably 

 due to the charging of the spark terminals to a sufficiently 

 high potential to break through the dielectric. The fact 

 that the insertion of a spark-gap into a secondary circuit in 

 the manner described has no effect upon the length of the 

 waves set up in that circuit was tested for a number of 

 different cases (in none of which, however, was the length of 

 the waves greater than in the present case), and found to be 

 true in each one of them. 



In order to determine the time of vibration we used a con- 

 cave rotating mirror; and the images of the oscillating sparks 

 were thrown on a sensitive plate. If the mirror rotated 

 about a horizontal axis, the photographs showed bright 

 horizontal lines, perpendicular to which at their extremities 

 extended two series of dots. Fig. 2 is an enlargement (about 

 ten times) of one of the sparks taken in our final measurement. 

 The dots represent discharges from the negative terminals 

 onlv, the positive discharges not being brilliant enough 

 to affect the plate. The distance between successive dots 

 was the distance on the plate through which the image of 

 the spark-gap moved during the time of a complete oscilla- 

 tion. Hence by determining the speed of the mirror and 

 measuring the distances from the mirror to the plate the time 

 of oscillation could be calculated. To measure the sparks we 

 used a sharp pointer moved at the end of a micrometer-screw 

 under a magnifying-glass of low power. The instrument was 

 originally intended for microscopic measurements, and was 

 verv accurately constructed. The rotating mirror was driven 

 by an electric motor by means of a current from a storage- 

 battery of extremely constant voltage. To give great steadi- 

 ness a heavy fly-wheel was attached to the axis of the mirror. 

 The speed of the mirror was determined to within about one 

 part in five hundred by means of an electric chronograph. 

 This apparatus, requiring great technical skill, was made for 

 us by the mechanician of the laboratory. The mirror con- 

 sisted of a thick piece of glass with a concave surface 

 accurately ground for this research by the well-known 

 optician Alvin Clark, and silvered by ourselves. For the 

 extremely sensitive dry plates which were used we are 

 indebted to Mr. Gustav Cramer, of St. Louis, Mo. 



There are many advantages in photographing the secondary 

 spark rather than the primary. In the first place, to properly 

 photograph a spark it is necessary to use pointed terminals ; 

 but experiment has shown that the waA r es excited in a secondary 



