23 
APPARATUS 
At the proper time, pressure on a button com 
pletes an independent circuit through the magnet 
seen below the segmented ring, figure 7, and in the 
side diagram of figure 8. 
The action of the armature releases the lower end 
of the rod on the loose collar, which, by means óf a 
coiled spring, is immediately thrown into gearing with 
the already revolving shaft; the contact brush sweeps 
around the segmented ring and effects the consecutive 
series of exposures at the pre-arranged intervals of 
time. 
At the University the intervals varied from the 
one-sixtieth part of a second to several seconds. 
A record of these time intervals was kept by a 
chronograph, a well known instrument; it comprises a 
revolving drum carrying a cylinder of smoke-black 
ened paper, on which, by means of successive electric 
contacts, a pencil is caused to record the vibrations of 
a tuning fork, while a second pencil marks the com 
mencement of each photographic exposure. The num 
ber of vibrations occurring between any two successive 
exposures marks the time. The tuning fork used 
made one hundred single vibrations in a second of 
time. To ensure greater minuteness and accuracy in 
the record, the vibrations were divided into tenths, and 
the intervals calculated in thousandths of a second. 
For the purpose of determining the synchronous 
action of the electro-exposors while making a double 
series of exposures, the accuracy of the time intervals 
as recorded by the chronograph, and the duration of the 
shortest photographic exposures used in the investiga 
tion, the two batteries of portable cameras were placed