313 



VI. Rheostat. 

 This piece of apparatus (Fig. 9) is 35 cm. long, 30 cm. high, and 15 cm. 

 wide. It has a wooden frame A, over which the wire B is tightly 

 stretched, the two sides being connected by the wire Q. The voltage, which, 

 as here used, was a constant one, enters at M through the binding posts 

 E E\ and from this it passes through the wire B in the direction indicated 

 by the arrow. The wire used was iron number 20 and in all 100 meters 



Figr. 9. 



were used in making this machine to obtain if needed a high resistance so 

 that by means of shunting any strength of current may be obtained. If an 

 insulated wire F is connected with one post on an electric slide as at G, 

 and the other post of this electric slide is joined to another insulated wire 

 I, about the center of which is interpolated a milli-ampere meter J; then 

 by shunting with the free end K of the wire I to the non-insulated wires 

 B, an electric current if sufficiently strong will pass through a specimen 

 laid on the slide at L under the microscope P. If at first the current at 

 B is not strong enough, the free end of the wire K can be moved from B 

 in the direction of the an-ow till a current of the desired strength is ob- 

 tained. The strength of the current will be registered by the meter J. 

 In the experiment I tried, with 110 volts entering at E E^, a current of .7 

 of a milli-ampere was sufficient to cause the movement in the protoplasm 

 in Elodea cells to cease. It began again in 20 minutes. The lamps 



..-a 



Sti. 



Fig. 10. 



