OF DENISON UNIVERSITY 



A Simple CIhronocraph. 



»9 



The apparatus illustrated on Plate B, Fig. 4, can be made in any 

 laboratory, and is for many purposes a good substitute for an expen- 

 sive chronograph governed by clock work. It consists of an electrical 

 pendulum a, suspended on a steel knife edge, which makes and breaks 

 circuit by carrying a platinum wire through a drop of mercury at b. 

 The same circuit contains an ordinary telegraph sounder with a long 

 bent wire attached, so that every depression of the armature makes a 

 dot on a long strip of smoked glass, which can be drawn slowly along 

 the table in front of it. A large turning fork, carrying a short piece 

 of very fine wire as a stylus, makes a tracing, simultaneously, about 

 one half a centimetre to one side. This fork is conveniently supjjort- 

 ed by being inserted into one end of a piece of wood e, which is 

 hinged ac/, to the table. The amount of pressure of the stylus is reg- 

 ulated by a wooden screw passing through c, and pressing against the 

 table. If the pendulum is adjusted to beat seconds and vibrated 

 through a rather large arc so that the sounder works with a sharp, short 

 click, the period of the fork may be very accurately determined. The 

 close agreement of a number of independent determinations indicates 

 the accuracy of which the method is capable. This is shown in the 

 following series of four successive experiments : — 



Average number of vibrations per second, 55- 06 



If it is desired to measure any other short interval, having thus 

 accurately determined the period of the fork, it is only necessary to 

 stop the pendulum, so as to complete the circuit jjermanently, and 

 then introduce a telegrajjh key into the line. Now the sounder will 

 register accurately each time the key is depressed, and the time be- 

 tween any two depressions may be determined by counting the num- 

 ber of fork vibrations included between the dots corresponding to 

 those depressions. If for any reason it is desired to have the seconds 

 marked also, the key for the occasional signals must be included in a 



