560 BELL SYSTEM TECHNICAL JOURNAL 



convenient instrument the means for comparing recurrent time phenomena 

 with an accuracy of a millisecond or two on a continuous chart which shows 

 the records for an entire week. Electrical impulses, related to the time 

 phenomena to be recorded, operate trigger tubes which discharge condensers 

 through the primary of an induction coil and cause sparks to jump from 

 a rotating spiral through a special chart paper having a dark colored backing 



Fig. 28 — Photomicrograph of single spark record showing nature of recording on wax- 

 coated chart paper. X 100 



and coated with a very thin layer of white wax. As the chart paper moves 

 slowly under the spiral, corresponding to the time abscissa, the succession 

 of sparks produces readily visible traces consisting of rows of tiny holes with 

 small areas around them where the wax is melted revealing the dark back- 

 ground. The holes are so small as to be scarcely visible, the darkened areas 

 constituting the visible trace. Figure 28 shows an enlargement of the 

 record of a single spark illustrating the nature of the marking. A recorder ^"'^ 

 very much like the Bell Laboratories' spark chronograph is used currently 

 as part of the standard frequency and time broadcast equipment of the U. S. 

 Bureau of Standards. 



