42 LAB OR A TOR Y G UWE IN PHYSIOLOG Y. 



rheocord is the same as that of the rheostat with the 

 exception that one ohm resistance is furnished by two 

 platinum wires which are stretched along the top 

 of the long resistance box. A mercury bridge 

 makes electric connection between these wires. When 

 the bridge or " slider " stands at the conditions are 

 the same as one has in the use of the rheostat with all 

 of the plugs in. As the bridge is moved gradually 

 from to 100, one ohm of resistance is as gradually 

 thrown into the short circuit. At that point a plug 

 representing one ohm resistance may be removed and 

 the bridge brought back to 0, and another ohm of re 

 sistance gradually introduced into the short circuit. 

 In this way any desired amount of resistance may be 

 introduced by infinitely small steps by infmitessimal 

 increments and the current of the long circuit will 

 be increased correspondingly. 



/. Appliances. 1 cell; Du B-R. Rheocord; detector; 5 

 wires; key. 



2. Experiments and observations. 



(1) Set up apparatus as shown in PI. II, Fig. 1. 

 With bridge at 0, close key and note angle. 



(2) Leaving the key closed gradually slide the bridge 

 to 1, then slowly and with an even rate of motion 

 on to 100, noting the behavior of the detector needle. 



(3) Open the key, remove the plug which represents 

 1 ohm, and slide the bridge back to the zero position, 

 close the key and note the angle at which the needle 

 comes to rest. If the resistance of the platinum 

 wires is 1 ohrn then the needle will come to rest at 

 the same point noted above when the bridge stood 

 at 100. 



(4) From this point the needle may be caused, by 

 sliding the bridge from to 100, to gradually in- 

 increase its angle. 



