132 PHYSIOLOGICAL PHYSICS, [Ciiap. xn. 



quickly contracts, and quickly relaxes, being helped 

 thereto- by the weight in the scale. By its relaxation 

 the points p and q will again make contact, and cause 

 the primary current again to be established. As the 

 oscillation period of the needle is great all this might 

 occur before the observation of the extent of the first 

 deflection, had been completed. It was to obviate this 

 that the arrangement of platinum plate and mercury 

 cup was devised by Helmholtz. The upper corner of 

 Fig. 66 shows what is meant. By the screw attached 

 to the forceps the muscle raises the lever so that the 

 platinum point just rests 011 the platinum plate. 

 Then, by means of the screw s, the mercury in the 

 cup is raised till q dips well into it. Then the 

 mercury is lowered by the screw till the point q and 

 the mercury in the cup are connected only by a thread 

 of mercury adhering to the amalgamated copper point 

 q, and sufficient to conduct the current. If by the 

 contraction of the muscle contact is broken, then, on 

 the return of the lever, the point q will no longer 

 make contact with the mercury, but will be separated 

 from it by a small space across which, before rupture, 

 the mercury thread stretched. A second shock to the 

 needle is therefore prevented. 



The wires that conduct the current from the 

 secondary coil may lead it directly to the muscle, one 

 being attached to the forceps, and the other to the 

 end of the muscle, or to a screw in the table, as in- 

 dicated by dotted lines in Fig. 67. But instead of 

 stimulating directly in this way, the nerve may be 

 left in connection with the muscle, and stretched over 

 two platinum electrodes at two different places, a and 

 b. By means of a commutator c, wit/tout the cross, 

 it can be arranged to send the stimulating shock to a 

 or to b at pleasure. Let it be sent to b, and take 

 the reading of the deflection of the galvanometer 

 needle that gives the latent period. Then reverse the' 



