168 PHYSICS. 
and the current, for an instant interrupted, immediately sets in again in the 
opposite direction, thus reversing the magnetism of the poles C and D. 
Respectively of like character with A and B, they are now repelled, and. 
thus driven round in the same direction until a recond reversal of their 
poles results in a second attraction. A spur wheel is attached to the upper 
extremity of the axis of the inner electro-magnet, which sets in motion the 
other apparatus shown in the figure, for the purpose of raising a weight. 
We may remark, in conclusion, that electro-magnetism has not answered 
the expectations formed of it as a motive power, the cost being as yet too 
vreat. Experiments are now in progress, however, which may result in 
showing its applicability to many purposes. 
The electric, or more properly electro-magnetic telegraph, is perhaps 
the most important result of the rapid communication of galvanism 
through conducting wires, and its electro-magnetic properties. It was 
first proposed by Ampére about 1823 to be operated by means of gal- 
vanometers. In 1825 Professor Barlow of Woolwich made a series of 
experiments, and found that the power diminished so rapidly with the 
distance that he pronounced the scheme impracticable. The next step in 
the discovery was made by Sturgeon about the same time. He bent a 
thick iron wire in the form of a horse-shoe, and rendered it magnetic by a 
galvanic current. Nothing further was done in reference to this subject 
until Professor Henry’s experiments in 1830, published in 1831 in Silliman’s 
Journal. He repeated the experiments of Barlow with tne galvanometer and 
single battery, and found the same result. He next substituted for the galva- 
nometer an electro-magnetic magnet, and again obtained similar effects. 
He afterwards changed the form of the battery and used one of intensity, 
and then found that the electro-magnet could be made to act at a distance, 
and announced the applicability of these results to the formation of 
the electro-magnetic telegraph. He also gave an account of two kinds 
of electro-magnetic magnets, both of which are now employed in the 
magnetic telegraph, one to be used with the single battery, formed of 
a number of short strands of copper wire, and the other in the long circuit 
with a compound battery, and formed with one long wire coiled around the 
magnet. The first, or a modification of it, is now employed as the relay 
magnet, and the second is the magnet of the long circuit. 
Referring our readers to professed works on the subject for additional 
facts in the history of the electro-telegraph, we proceed directly to an 
explanation of some of the principal forms that have been suggested and 
employed in various parts of the world. Of these, the first to be mentioned 
is Wheatstone’s telegraph (pl. 22, fig. 40). Two horse-shoe electro- 
magnets are fastened to a board, and wound with copper wire insulated by 
a silk wrapping. One extremity of the wire wound around the left horse- 
shoe passes under the board to the brass post a, the other goes to the post b. 
Other wires are screwed to these posts, passing to a point at some distance 
from the horse-shoes, where there is a galvanic battery. On bringing these 
conducting wires last mentioned into communication with the opposite poles 
of the battery, the left horse-shoe will become magnetic, this magnetism 
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