Trowbridge and Duane— Velocity of Electric Waves. 107 
of the primary circuit was increased by about 20 per cent. of 
its value, and a second photograph taken. In the first case 
the distances between successive dots were all within 2 or 3 
per cent. of the average obtained, by measuring over several 
dots and dividing by the number of intervening spaces, whereas 
in the second case the measurements of some of the single 
spaces were from 8 to 12 per cent. greater, the average from 
long measurements being the same as before. This indicates 
that the vibrations of the secondary circuit are not perfectly 
regular, and at a distance apart fixed by the character of the 
wire, but are to be looked upon as a series of pulses 
traveling along the circuit and keeping at a distance from each 
other, which is determined by the exciter. Owing to the fact 
that the damping of the primary is much greater than that of 
the secondary, the seventh and eighth pulses started are too 
weak to obliterate the first and second, which have traveled the 
length of the circuit and back. We should expect from this 
that the bolometer throws, which measure the average length 
of the wave, would not indicate a shifting of the node, when 
the circuits are thrown slightly out of resonance, but that the 
minimum throws would be greater than when the circuits are 
exactly in resonance. This, as is well-known, is what happens. 
_ The improved sparks, which the new arrangement of appa- 
ratus, and the use of cadmium as material for the spark ter- 
minals have enabled us to produce, have brought to light 
another interesting fact, namely, that even when the best 
resonance is obtained, and the most regular wave formation is 
excited, the distances between the first three or four dots are 
slightly greater than the distances between three or four dots 
taken farther down the spark. The explanation we offer for 
this is the following, and it applies as a criticism to all cases 
in which waves are excited in a cireuit by a neighboring cir- 
cuit possessing a much larger damping factor. The fact that 
the secondary waves last longer than the primary oscillation 
means that the last times the waves travel over the circuit, 
they do so under different end conditions from the first few 
times. The capacity of the secondary plates is slightly less 
after the primary spark has stopped than it was before, and 
therefore the length of the wires equivalent to the secondary 
plates is slightly less, and it takes a shorter time for the waves 
to travel along the circuit and back. Hence the observed de- 
crease in the distance between the spark points and a certain 
mixing up of the dots, which occurs after the sixth or seventh 
oscillation. (See figure.) The sixth dot in the figure, appar- 
ently following its predecessor after about half an interval, is 
not a usual characteristic. In the vast majority of sparks the 
first few dots are far more powerful than those that follow 
