106 Trowbridge and Duane— Velocity of Electric Waves. 
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 consisted of a thick piece of glass 
with a concave surface accurately ground for this research by 
the well known optician Alvin Clark. For the extremely 
sensitive dry plates which we used, we are indebted to Mr. 
Gustav Cramer of St. Louis, Mo. 
Upon photographing the secondary spark some curious phe- 
nomena were observed. In the first place the dots usually 
appeared in pairs. ‘There would be two black dots followed 
by a space, then two or three dots either appeared faintly or 
were absent altogether; after which the two black dots would 
reappear followed again by a faint space, and so on for six or 
seven repetitions. All this, of course, occurred in a single 
spark. The explanation that first presents itself is that the 
two black dots are the results of the first two oscillations in the 
primary circuit, which, owing to the damping, are much more 
powerful than the others. If this were the true reason, the 
first of the pair of dots always ought to be blacker than the 
second, and every third dot ought to be the first of a pair. 
This is not the case, however. On the other hand, the phe- 
nomena cannot be explained as the result of a complex vibra- 
tion, for the bolometer readings, taken only a few minutes 
before the photograph plates were exposed and with exactly 
the same arrangement of apparatus, indicated extremely regu- 
lar waves. A clue to the mystery was furnished by several 
sparks, in which the dots made by one spark terminal had the 
characteristics just described, whereas those made by the other 
were quite regular. [Following out this hint, we found that the 
ee substance used for the secondary spark terminals 
ad a large effect upon the characteristics of the photographs. 
We tried spark terminals made of a number of different metals 
—tin, aluminum, magnesium, fuse-metal, ete., and finally 
adopted cadmium as productive of the best sparks. In the 
case of cadmium the characteristics described are much less 
marked, and we have succeeded even in producing a few 
sparks in which no difference in blackness could be detected 
between one dot and the next. The photographs from ead- 
mium terminals, too, are far more distinct, and far more easily 
measured, than those from terminals of any other metal that 
we tried. 
An interesting question arose here, as to whether the dis- 
tance between two successive dots would depend upon the 
period of oscillation of the primary circuit, if the secondary 
were unaltered? To test this point, the circnits were brought 
into resonance, and a photograph taken. The self-induction 
