660 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



electro-magnet. The following is one of the solutions of the problem 

 he has proposed, and his system is at work on several lines. 



The axle-tree A of the waggon carries an excentric c, which 

 produces the reciprocating motion of the crank B and the oscillation 

 of a shaft o, attached to the crank by one arm of a lever. This shaft 

 also carries a lever e, whose extremity has a tongue of soft iron p, 

 which places itself at each oscillation opposite the poles of an electro- 

 magnet E. As long as the current is not thrown into this magnet 

 it has no power of attraction, and it remains hanging by the rod 

 which carries it ; but if the brakesman, by means of a commutator 

 within reach, closes the circuit of the battery, immediately the 

 electro-magnet and the tongue are in magnetic contact, and both 

 oscillate together. The suspending rod of the electro-magnet carries 

 a catch K, which is kept by a spring r against the toothed-wheel B ; one 

 of the eight teeth of this wheel is thus pushed on at each oscillation, 

 the wheel turns through an eighth of its circumference, and with it 

 the mechanism which actuates the brake. 



We need not here describe the brake itself, it is sufficient for our 

 purpose to show how the throwing it into and putting it out of gear 

 are effected by the passage or interruption of an electric current 



TV. MAGNETO-ELECTRIC MACHINES** 



Few discoveries in physical science have been more important in 

 themselves, or richer in practical results, than Faraday's discovery of 

 the induction of electrical currents. QErsted's grand discovery, which 

 linked together electricity and magnetism, had already yielded a 

 scientific harvest of uncommon richness. It led immediately to 

 the construction of electro-magnets vastly exceeding in power any 

 permanent magifets which were then known or have since been made. 

 The multiplier or galvanometer of Schweiger supplied a new and 

 important instrument for measuring electrical currents, which with a 

 little modification became the electric telegraph. Faraday discovered 

 the rotatory character of the reciprocal action of magnets and electrical 

 currents ; and Ampere showed that all the properties of a permanent 



1 Condensed from the report of a lecture, delivered before the Belfast Philoso- 

 phical Society, by Dr. Andrews. From Nature, June, 1875. 



