PHYSIOLOGICAL PHYSICS. [Chap. vin. 



plates. These holes exist between all the plates, 

 except between the first two, and when all the plugs 

 are inserted the separated brass plates become, so far 

 as the conduction of a current is concerned, one con- 

 tinuous brass plate. From the first plate of brass at 

 a there runs a platinum wire, over one metre long. 

 It goes nearly to the other end of the block of wood, 

 and terminates at a screw at b, after passing over an 





10 





Fig. 42. Rheocord of Du Bois-Reymond. 



ivory knife ed^e. From the second plate at c another 

 similar wire runs parallel to the first, ending at d. 

 Stretching along the side of the block of wood from ac 

 to the ivory knife edge is a raised rail of wood, which 

 supports a little brass platform s, the one being dove- 

 tailed on to the other, so that they cannot be separated, 

 but so that the platform can slide along the rail from one 

 end to the other. The platform, or slider, as we shall 

 now call it, carries two little hollow cylinders of steel 

 shaped like conical bullets, with the pointed ends 

 directed to the brass plates. The cylinders are filled 

 with mercury, and closed at the wide end by corks. 

 The platinum wires pass through them by means of a 

 little hole in the pointed end and a small opening in 

 the centre of the corks. When the slider is brought 

 close up to the brass plates, the pointed extremities of 

 the cylinders make contact with the first and second 

 plates, between which, as already noted, there is no 

 space cut out for a connection by a brass plug. The 

 slider, therefore, establishes the connection, forming 

 by its steel cylinders in contact with one another and 



