of Electric Chronograph. 379 



The rails. — Two steel rails, A B and C D, each 2 metres long 

 (Plate IX. fig. 1), are attached to three cast-iron standards : 

 two of these, M and N, are shown. Each standard is fur- 

 nished with two le veiling-screws. The upper rail is adjustable, 

 so that it may be placed parallel to the bottom rail, and at a 

 correct distance from it for the width between the wheels of 

 the carriage. (The rails are slightly inclined from D to C.) 



The carriage, JE F, is built up of bars (which are not shown) 

 so as to be both light and rigid. It runs upon three wheels, 

 each wheel being carried on two steel points ; the points are 

 adjustable, so that the face of the carriage can be placed 

 parallel to the plane of the rails. To the front of the carriage 

 a sheet of smoked glass is fixed with clips and screws. The 

 carriage is furnished with a catch which engages with a detent 

 at H ; by this it is held back against the pull due to the weight, 

 W, until required to run past the styli. A projection at the 

 back of the carriage engages with a leather brake-band. 



The brake. — A band of leather, L, is fixed to the standard, 

 M, and also to a strong spiral spring at the back of the upper 

 rail : the projection previously mentioned rubs under the band 

 and brings the carriage to rest without any concussion. 



The weight. — A gut-band or cord, passing over the pulleys, 

 Q, P, R, is fixed at G ; it is provided with a small ball which 

 engages with a fork fixed to the back of the carriage. The 

 weight impels the carriage until it is arrested by contact with 

 an adjustable table T; the carriage then runs on with the velo- 

 city acquired. The velocity is found to be practically uniform 

 throughout the whole length of a time-trace. The cord, if 

 free, has a velocity approximately double that of the driving- 

 weight. 



Electromagnetic styli. — These are shown at A A (fig. 2). 

 An electromagnet of peculiar construction is mounted upon a 

 brass plate, which carries a T-shaped lever ; to this a writing- 

 point of aluminium, mica, or parchment-paper is attached. 

 The lever is held by the electromagnet, as long as a current is 

 acting upon it, against the pressure of a spring : when the 

 current is broken the lever rises and gives its marking. The 

 electromagnet is so constructed and wound that its period of 

 " latency ' is exceedingly small, and also constant within 

 close limits. By means of a flat rectangular spring the stylus 

 is attached to a pillar furnished with a sli ding-holder B : each 

 stylus can be easily adjusted to a vertical line for experiments 

 in which a large number of markings are required in succession. 

 The pillar-support. — It has been found that, unless the sup- 

 port of the styli is free from vibration, the trace produced by 

 them is not worth much. The pillar used in connexion with 



2G2 





