194 ELEMENTARY GENERAL SCIENCE CHAP. 



unconnected the current is said to be open, and no electric 

 current circulates. 



Work done by the Current. The only work which the 

 electric circuit performs in a closed circuit, such as is shown 

 in Fig. 94, is of a useless kind, for it is completely used up in 

 warming the connecting wire and the liquid. 



EXPT. 18V. Connect the wires from the carbon and zinc 

 poles of a Bunsen's battery to the ends of a thin piece of 

 platinum wire. Notice that when the circuit is closed the 

 platinum wire becomes red hot. 



But there are many kinds of useful work which such a current 

 can perform. 



EXPT. 188. Fit up a Bunsen's cell, taking care that there is 

 a bright metallic connection at each binding screw. Join the 

 wires from the carbon and zinc poles respectively to the two 

 binding screws of an electric bell. Notice the ringing. 



EXPT. 189. Similarly attach the wires to the binding screws 

 of an electro-magnet. Notice (a) its lifting power, by attach- 



FIG. 101. An Electro-Magnet with Armature. 



ing gradually increasing weights to the hook of the armature ; 

 (6) remove the armature, support the electro-magnet vertically, 

 with the poles or ends of the magnet upwards, and place a 

 sheet of cardboard, or a plate of glass, on them. Sprinkle 

 fine iron filings evenly over the cardboard or glass, gently 

 tap the latter, and notice their arrangement in lines, which 

 proceed radially from the poles, and which are called lines of 

 force (Fig. 102). 



