VI. Demonstration: To vary the current through 



the use of (a.) the simple rheocord, or (b.) 



the Ludwig compensator. 



Besides the methods already used for varying the 

 strength of the current one may use the derived current. 



The simple rheocord (Fig. 7) may be used for this 

 purpose. 



a. The simple rheocord. 



/. Appliances, One or more cells; simple rheocord; 5 wires; 

 detector. 



FIG. 7. 

 FIG. 7. The simple rheocord. See also PI. II, Fig. 2. 



2. Experiments and observations. 



(1) set up the apparatus as shown in Fig. 2, Plate II. 

 From the figure we see that from the cell to post 

 A, thence through the German silver wire to postB 

 and back to the cell makes a complete circuit. Hav- 

 ing reached the metallic slider (S) the circuit has 

 two paths presented. 1st, from S direct to B; 2d, 



43 



