32' THE MECHANISM OF 



dulum, etc.) being too slow for such short exposures, a new method 

 was devised, in which the moving body was a circular black disk 

 over five feet in diameter, turned by a crank with a multiplying 

 gear of ten to one, so that for a single turn of the crank there 

 were ten revolutions of the disk, and at high speed the chances of 

 irregularities in the motion were therefore reduced to a minimum. 

 A cam on the axle of the disk made an electrical contact between 

 two metallic springs at each revolution, by means of which the 

 speed of each turn was recorded on the drum of the chronograph. 

 On the black surface of the disk, near its periphery, was a white 

 spot of a certain width ; its distance from the centre, and (from 

 that and the chronograph record) the distance which it passed over 

 in the time of one revolution, were accurately known. From the 

 increased wudth or blur of the spot shown in each photograph the 

 duration of that exposure was calculated ; and by noticing the 

 difference in the position of the spot in any two consecutive pho- 

 tographs of the series the interval between successive exposures 

 was determined, it was seen whether they were all equal as they 

 should be, and whether the intervals determined in that way agreed 

 with those recorded on the chronograph by the style in circuit with 

 the contact-motor. 



In making the experiment the two batteries of small cameras 

 were placed side by side opposite the disk ; they were arranged 

 for the most rapid exposures ever used in the investigations, and 

 the contact-motor was adjusted so that one battery should be dis- 

 charged after the other, a series of twenty-four consecutive ex- 

 posures in about one second. When all was ready the contact- 

 motor and the chronograph were started, and the disk was re- 

 volved at a speed of about two revolutions per second ; the switch 

 in the chronographic circuit of the disk was thrown in, and simul- 

 taneously the contact-key was pressed, and the series of expos- 

 ures were made. A portion of the very interesting chronographic 

 record of the series is reproduced in Fig. 12, the upper line re- 

 cording the revolutions of the disk, the middle one in the vibra- 

 tions of the tuning-fork, and the lower one the intervals between 

 exposures. The vibrations in the upper and lower lines, after the 

 drop caused by the passage of the current, were due to the neces- 

 sarily long styles in those two electro-magnets, and are not so 

 noticeable in the regular two-line records of the other work. 



