.THE ELECTRO-MAGNET. 



with any desired degree of rapidity; and, by varying the power of 

 the battery, the number of coils of the spiral wire, and the mag- 

 nitude of the iron bar, a magnetic force of any desired intensity 

 can be produced. 



A piece of iron, called an armature, is presented to one or both 

 of the poles of the magnet towards which it is attracted, while the 

 current is transmitted with a force proportionate to the intensity of 

 the magnetism ; and when the current is suspended, the armature 

 either falls .from the magnet by its own weight, or is withdrawn 

 from it by the action of a spring, or other mechanical expedient, 

 provided for the purpose. 



The armature may be placed between two magnets, which are 

 alternately acted upon by the electric current, which is trans- 

 mitted round each in the intervals [of its suspension round the 

 other. The armature will then be moved alternately to and fro 

 between the two magnets. 



6. In this manner, by alternately suspending and transmitting 

 the current on the wire which is coiled round the electro-magnet, 

 the magnet and its armature receive an alternate motion to and 

 from each other, similar to that of the piston of a steam-engine, 

 or the foot of a person who works the treddle of a lathe. This 

 alternate motion is made to produce one of continued rotation by 

 the same mechanical expedients as are used in the application of 

 any other moving power. 



The force with ;which the electro -magnet and its armature 

 attract each other, determines the power of the electro -motive 

 machine, just as the pressure of steam on the piston determines 

 the power of a steam-engine. This force depends on the nature 

 and magnitude of the galvanic pile which is employed. 



7. The pile used by M. Froment for the lighter sort of work, 

 such as that of driving his engines for dividing the limbs of 

 astronomical and surveying instruments, and microscopic scales, 

 is that of Daniel, consisting of about twenty-four pairs. Simple 

 arrangements are made by means of commutators, reometers, and 

 reotropes, for modifying the current indefinitely in quantity, 

 intensity, and direction. By merely turning an index or lever in 

 one direction or another, any desired number of pairs may be 

 brought into operation, so that a battery of greater or less inten- 

 sity may be instantty made to act, subject to the major limit of 

 the number of pairs provided. By another adjustment, the copper 

 elements of two or more pairs, and at the same time their zinc 

 elements, may be thrown into connection, and thus the whole pile, 

 or any portion of it, may be made to act as a single pair, of 

 enlarged surface. By another adjustment, the direction of the 

 current can be reversed at pleasure. Other adj ustments, equally 



