MAGNETIC EFFECTS OF ELECTEIC CURRENTS 229 



235. Uses of small electromagnets. Electromagnets made 

 by winding insulated wire about an iron core were first 

 made in 1828 by Joseph Henry (1797-1878) of Albany, 



~ FIG. 114. Small electromagnets 



The two at the left are small lifting magnets for experimental work ; the middle 

 object shows the electromagnet of a telegraph sounder ; at the right is the elec- 

 tromagnet of a toy motor 



New York. Many thousands of magnets of this type are now 

 manufactured annually. The range of size is from the tiny 

 coils in a telephone receiver to the giant lifting magnet 

 which handles several tons of iron or steel at one lift. 



FIG. 115. Electromagnets of a telephone 



At the left is the receiver with its magnets at H, fastened to the wires G, inclosed 



in the casing, F, and the iron covering plate, 7; at the right, the magnets and dry 



cells of the bell ringer 



The telegraph sounder (fig. 114) consists of two coils 

 having a resistance of from 4 to 20 ohms. When the iron 

 cores in the coils are magnetized by a direct current from 



