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XIX. An Instrument for Measuring Chronograph Traces. 

 By Frederick J. Smith, M.A., Millard Lecturer, Mech. et 

 Phys., Trinity College, Oxford*. 



HEX a tuning-fork is used to determine the period of 

 time between two chronograph markings, traces 

 such as A B, Si, S 2 (fig. 1) are produced on a sheet of smoked 



w 



c 





Fig.: 



E 



L. 







a/ m 



T> 



X 



-$ 



T 



P B 





r 









Sx 











r 





glass, which is carried in contact with the points of electro- 

 magnetic styli and the stylus of a tuning-fork. In the 

 figure, S x S 2 are the markings of the electromagnetic styli, 

 they show the beginning and ending of a time period : 

 parallel lines ruled through S x S 2 cut the fork trace at M, X. 

 The central line A B is ruled through the curved trace 

 C D E F by the stylus of the fork when at rest, in its position 

 of equilibrium. Hitherto I have measured a length such as 

 t, which is made up of one whole vibration M L and a frac- 

 tion of a vibration L X, by means of a traversing micrometer- 

 microscope, furnished with a spider's web, or a quartz fibre, 

 in the eye-piece. This method of determining the length of 

 LN in terms of a vibration-length, though very accurate, has 

 the disadvantage of taking a considerable time to go through. 

 By means of a device which I have called the Wedge-Divider, 

 this value may be accurately and quickly determined. A 

 triangle (fig. 2) of which the sides A G, B C are equal is 

 ruled on a piece of plate-glass ; the base A B is divided into 

 any number of equal parts, and each division is joined to C by 

 straight lines. I usually divide A B into 100 parts, and each 

 tenth line is dotted as well as ruled. To use the wedge-divider, 

 the trace is removed from the chronograph, and after being 

 ruled, is varnished with very thin photographic varnish. The 

 divider is then placed upon the trace with its ruled side in con- 

 tact with the trace, and moved at right angles to the trace till 

 A C and B C cut the trace in X and M, the ends of a vibration 



* Communicated by the Author. 



