326 Mr. R. Sabine on Measuring the Contour of 



of spring e with stud g and the breaking of contact be- 

 tween / and m is shown by the engraved division upon which 

 the index of the arm h stands. The disk, making two revolu- 

 tions per second, each of the 500 divisions necessarily repre- 

 sents xoVo °f a second, which is further divisible by the nonius 

 to 10 q 00 . It is of course assumed that the rate of rotation 

 during a revolution is uniform, the retardation through friction 

 being too minute to be appreciable in so short a time. 



The original way of "timing" this apparatus was to start 

 it and note the figures read in succession under the fibre of a 

 small telescope when the disk was illuminated by half-second 

 flashes of an induction-coil. The force of the spring and the 

 position of the trigger were adjusted until the right velocity 

 was obtained. Recently I have employed an equally accurate 

 and easier way of "timing," by discharging an accumulator 

 through a known resistance for some interval indicated by the 

 disk, and calculating, from the initial and final readings of the 

 galvanometer, the time according to the leakage-formula. If 

 the^ two do not agree, I alter the spring until they do. The 

 spring, however, is very constant ; and no adjustment has been 

 found to be necessary, except after taking the apparatus to 

 pieces for alterations. 



The manner in which the apparatus is connected up is shown 

 in plan in fig. 4. The body of the disk d d is connected with 



Fig. 4. 





end a of the conductor (acb). A contact g f , against which 

 the spring e presses, puts the battery B' to the line when the 

 disk is trigged ; and another contact, #, puts in the opposite bat- 

 tery B, when it is released. This arrangement is used when 

 the neutral point of two opposite waves is to be found ; but 

 when the contour of a single wave is required, the battery B / 

 is omitted and g f connected to earth in order to facilitate the dis- 

 charge of the line. The electricity of the point c goes through 

 m and I to A, which it charges. When the disk rotates, the 



