APPARATUS 23 



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 of 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 (vas 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 



