LABORATORY GUIDE IN PHYSIOLOGY. 



b. The Ludwig compensator. 



This instrument, though used in a. class of experi- 

 ments quite different from those in which the rheocord 

 is used, involves the same principle as that involved 

 in the simple rheocord, and is used to make minute 

 variation in the strength of a current. The general 

 construction of the instrument is shown in Fig. 8. 



A 



FIG. 8. The Ludwig 

 compensator, originally 

 devised by Ludwig to 

 compensate a muscle 

 current, may be used 

 in the same way as the 

 simple rheocord. Its 

 maximum current is, 

 however, limited. For 

 description, see VI=b. 



FIG. 8. 



The outer receptacle is of copper and serves as the 

 copper plate; within is a porous cup containing the 

 zinc plate. This is practically a Daniell cell. A 

 graduated upright of brass makes metallic contact 

 with the copper plate, and at A the circuit is com- 

 pleted by a platinum wire to B. 



