V. Demonstration: Methods of varying the strength 



of current, a. The rheostat, b. The 



Du Bois=Reymond rheocord. 



It has already been shown that the strength of current 

 may be varied by increasing the number of cells or by 

 changing their arrangement in the battery. This method 

 is indispensable, but it has its limitations. If one has a 

 small cell and wishes to decrease the current, he must 

 have recourse to another method. From the formula C = 

 -|- it is evident that one may decrease the current by in- 

 creasing the resistance. 

 a. The rheostat. 

 /. Appliances. Resistance box or rheostat; 1 cell; 5 wires; 



detector. 

 2. Experiments and Observations. 



(I) Set up the apparatus as shown in PL I., Fig. 6. 



(1) With plugs all fixed in rheostat, needle of detec- 

 tor at 0, close key and note angle of deviation. 



(2) Remove the plug which will throw into the circuit 

 the lowest resistance contained in the rheostat. 

 Note the angle. 



(3) Add to the above resistance the smallest possible 

 increment and note angle. 



(4) Proceed in this way tabulating results. 



(5) Conclusions. 



(II) Another method of using the rheostat. The rhe- 

 ostat may be used in short circuit as shown in PI. I., Fig. 

 9. From this arrangement of the apparatus it is appar- 

 ent that when all of the plugs are in place the current 

 will be short circuited by the rheostat. If the resist- 

 ance of that part of the circuit leading to the detector 

 the long circuit be considerable the long circuit 



40 



