338 LABORATORY GUIDE JN PHYSIOLOGY. 



4. THE MAREY TAMBOUR. -See Fig. 4. 



FIG. 4. 

 5. THE POHL COMMUTATOR. See Fig. 5. 



FIG. 5. 



6. THE INTRODUCTION COIL OR INDUCTORIUM. Figure 6 shows 

 DuBois-Reymond's instrumen). Ludwig's instrument consisted in 

 changing the axia of the coils to the vertical position and counterpoising 

 the secondary coil. The DuB-R. instrument, or some modification of it, 

 is in more general use, and is satisfactory. 



