Electricity. 329 



to be due to electric currents flowing around the earth from east to west. 

 These currents are produced by the variations of temperature, which 

 take place even r where daily in consequence of the revolution of the 

 earth on its axis, showing new parts constantly to the sun. If a mag- 

 netic needle be brought near a wire carrying a strong current of elec- 

 tricity, it will tend to place itself at right angles to the current. If an 

 experimenter should hold the wire in his hands and above the needle, 

 the current running from left to right, the positive pole of the needle 

 would point away from the person holding the wire. If the wire were 

 held under the needle it would point towards the holder. If the cur- 

 rent ran in the opposite direction the phenomena would be reversed. 

 The reverse of this experiment is equally true; viz., if the magnet be 

 fixed and the wire carrying the current be free to move, it will tend to 

 place itself at right angles to the magnet. Fig. 139 represents an ap- 

 paratus consisting of a hoop of wire suspended upon points which are 



FIG. 140. 

 A Solenoid. 



FIG. 139. 



inserted into cups containing mercury, so that it is free to revolve with- 

 out disturbing the connection. If this hoop be placed so that its plane 

 coincides with the magnetic meridian of tha earth, and then a current of 

 electricity be passed through it, it will turn on its suspending points 

 and place itself at right angles to the meridian, and take such position 

 that the current in the lower part of the hoop will flow from east to 

 west, the same direction taken by the surface currents of the earth. 

 Fig. 140 represents a solenoid. This consists of a wire coiled into a 

 double helix and suspended by points, upon which it can turn. When 

 a current is passed through the helix it acts in the same way as a mag- 

 netic needle, taking a north and south position in such a way that the 

 current in the bottom part of each coil is flowing from east to west, so 

 that the end marked ( a ) becomes the north pole and points to the north. 

 Two parallel currents going in the same direction attract each other 

 bodil} T , that is, the wires carrying them tend to approach each other. 

 If the currents are in opposite directions they repel. In two hoops of 

 wire made to float and convey currents, if both go alike, either in the 

 direction of the hands of a watch, or the contrary way, they will move 



