92 DYNAMO-ELECTRIC MACHINES. 



and the wire be wound so that its motion throught the field will be 

 perpendicular to the lines of force, and parallel to the axis. The 

 electro motive force, as well as the intensity of the current, will de- 

 pend upon the length of the conductor and the nnmber of lines of 

 force cut in a unit of time. With a uniform field, that is one in- 

 which the magnetism remains constant, this varies simply as to the 

 speed. As magneto-electric machines always create a uniform field, 

 the electro motive force in such machines varies directly as the speed. 

 Having thus endeavored to explain the principles- governing 

 the production of currents in dynamos, their action may be stated as 

 follows : — First, it will be understood that all metals capable of 

 being magnetized always possess previous to being acted upon, and 

 always retain afterwards, a certain amount of magnetism. It is im- 

 material whether this varies with different metals, or not. This is 

 called residual magnetism, and by electricians is stated to be due to 

 the influence of the earth, which by them is considered a great 

 magnet, with its two poles north and south. Secondly : the brushes 

 or collectors, which collect the electricity from the commutator, are 

 usuaUy joined, the positive to one of the terminals of a pair of coils, 

 if the electro-magnets are double, and to one, if single, after traver- 

 sing the conductor, of which these may be formed, and thence to the 

 lamps for lighting purposes, or the electro-chemical baths if for 

 plating purposes, or to other dynamos if for the transmission of 

 power. The current is then returned through the negative conduc- 

 tor to the terminals of the remaining electro-magnet coils, and thence 

 is connected to the negative collector. When the armature is set in 

 motion, the residual or permanent magnetism of the field magnets 

 generates a feeble current, which passes out by the commutator 

 around through the first coils, and the whole of the circuit back 

 through the other coils, and so to the other brush, and in its passage 

 increases the magnetic action of those magnets, which in turn gener- 

 ates an increased current in the armature wire, which in its turn 

 causes increased energy in the electro-magnets, and so acts and 

 reacts until it attains its maximum. This requires but a very short 

 period of time. The time is so short that it cannot be measured by 

 seconds, but by fractions of a second. The plan spoken of, of 

 sending the current through one-half of the coils forming the electro 

 magnets, and thence to perform whatever duty is required of it, 



