Description of an Electro-motive Engine. 179 



keeper never adheres to the magnet ; it is instantly repelled, 

 and this rapid alternate movement is made to act on a fly 

 wheel. 



The annexed figure (II.) shows the manner in which the 

 magnet A. and keeper B. are arranged. The keeper is a tri- 

 angular prism, resting on two bits of brass projecting under 

 the magnet. The movement is evident from the figure ; the 

 lower edge of the keeper never quits the magnet. 



Both the magnet and keeper are electro-magnets ; they 

 are made to attract and repel by the same current without 

 altering its direction. The mode of effecting this is as follows : 



The magnet and keeper communicate with the battery 

 alternately, and the motion of the keeper causes the current 

 to pass from the one to the other. 



Let us suppose that the magnet being in communication 

 with the battery, attracts the keeper till they come in con- 

 tact. If, at this instant the current is suddenly shifted from 

 the magnet to the keeper, the latter is repelled by the residu- 

 um of magnetism remaining for a moment in the latter, both 

 having then like polarity. When the keeper is thus thrown 

 off, on arriving at the desired distance (or angle) from the 

 magnet, it causes the current to be thrown on the magnet 

 again, and so on alternately. It is absolutely necessary that 

 this movement should be instantaneous, and be performed 

 with the utmost precision and accuracy; conditions, it will be 

 seen, very easily fulfilled, by means of the apparatus repre- 

 sented in the annexed diagram, Fig. I. 



Mercury is not used in any part of this arrangement. It 

 would not answer the purpose of interrupting and restoring 

 the communication with the battery, because that manoeuvre 

 requires a degree of precision the use of mercury does not 

 admit of. The mercury soon becomes an amalgam of copper, 

 and drags a tail, which alone is sufficient to interfere with 

 the movement ; and this is more especially the case when the 

 engine is large, and the electrical current powerful. 



