44 



EXPERIMENTAL GENERAL PHYSIOLOGY 



(1) With plugs all fixed in rheostat, needle of galvanoscope at 0, 

 close key and note angle of deviation. 



(2) Remove the plug, which will throw into 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. 



FIG. 23 



(II) Another method of using the rheostat. The rheostat may 

 be used in short circuit, as shown in Fig. 23. From this arrange- 

 ment of the apparatus it is apparent that when all the plugs are 

 in place the current will be short-circuited by the rheostat. If the 

 resistance of that part of the circuit leading to the galvanoscope 

 the long circuit be considerable, the long-circuit current will 



FIG. 24 



Galvanoscope, composed of a single magnetic needle mounted over a graduated circle. The 

 two heavy copper wires which encircle the compass offer slight resistance to the passage of the 

 electric current. 



probably not be sufficient to cause any deviation of the galvanoscope 



needle; for the current varies inversely as the resistance (Coo __), and 



R 



if the resistance .of the short circuit (R') equals zero, then the current 

 of the long circuit (C) will be incomparably less than the current of 



the short circuit (C')-i.e., C:C':: ^:~. or C:C':: R':R; therefore, 



H H 



if R' = Q,C must equal 0. 



