Chap, xvi.] THE GRAPHIC METHOD. 171 



If a cylinder be caused to revolve at a constant speed, 

 and if a lever be brought up against it, and caused to 

 make a mark upon it, the time during which the lever 

 acted on the cylinder can be estimated by the speed of 

 the cylinder. Thus, if the cylinder revolved once in 

 the second, and the mark extended half way round 

 the cylinder, the lever must have acted during half 

 the time of revolution of the cylinder, i.e. a half 

 second. Or, if the cylinder revolve once per second, 

 and the space of the cylinder be divided by 100 

 vertical lines into 100 equal parts, then each part 

 represents graphically the y^th of a second, and 

 so on. 



This idea was speedily taken up and developed by 

 some French experimenters, and specially by Professor 

 Marey of the College of France. To render the 

 movement uniform he added to the revolving cylinder 

 the regulator used by Foucault in his determination of 

 the velocitv of li^ht. 



/ o 



The cylinders now used are generally made of 

 copper, and are turned by clockwork, regulated by a 

 Foucault's regulator. There are usually two or three 

 axes of different degrees of speed, on any of which the 

 cylinder may be pivoted, or there is an arrangement 

 for altering the speed without moving the cylinder. 

 A dial plate indicates the number of revolutions. 

 The cylinder is covered with paper smoked by a tur- 

 pentine lamp. Any marker brought against the 

 cylinder removes the soot and makes a white mark as 

 a record of its contact. 



Even without such a regulator, however, accurate 

 measurements can be made by the use of electro- 

 magnets. Fig. 84 shows such an instrument, which 

 is constructed on the same principle as Wagner's 

 hammer. (See page 42.) The current from a battery 

 entering by one binding screw passes round the bobbins 

 of wire, converting their soft iron cores into temporary 



