316 Prof. Helmholtz on the Methods of Measuring 



numbers 40,000, half-degrees, which were marked upon the 

 cylinder, each half-degree thus occupying the ^J^dth part of a 

 second. Now if the point make two successive marks upon the 

 cylinder, we know that in the interval between both marks, as 

 many half-degrees will have passed by the point as there are 

 forty-thousandths of a second in the interval. Thus far, indeed, 

 the principle was carried out before Siemens ; but the chief dif- 

 ficulty which long defied the ingenuity of experimenters, was to 

 cause the times of marking upon the cylinder to correspond 

 exactly with the times of commencing and ending of the process 

 the duration of which was to be measured, for example, with the 

 precise points of time when the projectile struck two successive 

 objects placed in its path. The point was caused to press against 

 the cylinder and to scratch it, and this was first effected by an 

 observer who moved a lever as soon as he observed the striking 

 of the projectile. The incompleteness of this method is at once 

 evident from what we have already stated regarding the inaccu- 

 racy of our senses. The mechanical transmission of the shock 

 of the ball against the body struck, without the intervention of 

 an observer, requires also time, as a shock is transmitted through 

 a solid body with measurable velocity. Breguet and Wheatstone 

 went much further by making use of a galvanic current for the 

 transmission of the effect. The point which makes the marks 

 upon the cylinder is attached to a small lever, which is held by 

 an electro-magnet, as long as the latter is magnetized by the 

 current which circulates round it. As soon as the circuit is inter- 

 rupted at any place, the marking-point falls upon the cylindrical 

 surface and describes a line. The interruption of the current is 

 caused by the ball tearing asunder a net of thin wire placed in 

 its path, and which forms part of the circuit. The arrangement 

 was for the most part such, that, immediately after the interrup- 

 tion of the current, it was again established and the point raised 

 from the cylinder. By the rupture of a second net a second 

 mark was made. However ingenious these instruments were 

 constructed, it was not possible to make measurements with them 

 to a greater degree of accuracy than the ^th of a second, inas- 

 much as the marking-point, after being freed from the electro- 

 magnet, required a certain time to fall upon the cylinder, and 

 the velocity of the fall with unequal magnetic strength is not, 

 as assumed by the inventors, constant, but variable. In this 

 way, the exactitude, which otherwise might be ascribed to the 

 principle of the rotating cylinder, was greatly damaged. 



To Siemens occurred the happy idea of doing away with all the 

 mechanical interventions, and of permitting the electricity itself 

 to mark the times. If even a feeble electric spark passes from 

 a fine point to a well-polished surface, it leaves behind a fine 



