BY MONTAGUE RHYS JONES, C.E. 77 



moved by a motion of translation to another part of the 

 uniform field, as many lines of force will be left behind as 

 are gained in advancing from its first to its second position, 

 and there will be no current generated in the coil ; but if, as 

 in this figure (Plate VII.), the coil be tilted in its motion across 

 the uniform field, or rotated round any axis in its own plane, 

 then the number of lines of force that traverse it will be 

 altered and currents will be generated. These currents will 

 flow round the ring coil in the positive sense. ' The positive 

 sense of motion round a circle,' I may here add, 'is here taken 

 as opposite to the sense in which the hands of a clock go 

 round.' If the effect of the movement is to diminish the 

 number of lines of force that cross the coil they will flow 

 round in the opposite sense. If the effect is to increase the 

 number of intercepted lines of force, if the field force be not 

 a uniform one, then the effect of taking the coil by a simple 

 motion of translation from the place where the lines are more 

 dense to a place where they are less dense, as from position 

 1 to 2 on this diagram (Plate VIII.), will be to generate 

 currents, or if the motion be to a place where the lines of force 

 run in the reverse direction the effect will be the same, but 

 even more powerful." 



From the foregoing simple facts some very important con- 

 sequences are apparent, the principal of which are these : — 

 "Currents can be generated in conductors by setting up 

 magnetic whirls round them. We can set up magnetic whirls 

 in conductors by moving magnets near them or moving them 

 near magnets. This means energy and an expenditure of 

 power. The more rapid the motion the stronger the current, 

 and by using a suitable commutator or guide all the currents, 

 direct or inverse, produced during recession or approach can 

 be turned into the same direction in the wire that goes to 

 supply currents to the external circuit, thereby yielding an 

 almost uniform current." 



A very large number of dynamo-electric machines have 

 been constructed on the foregoing principles, and the variety 

 is legion, but the ones most used for the purpose of electric 

 traction are those in which there is a rotation of a coil or 

 coils in a uniform field of force, such rotation being effected 

 (as in this diagram — repeat Diagram VII.) round an axis in 

 the plane of the coil or one parallel to such an axis. 



An electric motor is the reverse in its action to the dynamo, 

 the latter converting mechanical energy into electrical force, 

 and the former conversely transforms the current of elec- 

 tricity supplied by the dynamo from electrical force again 

 into mechanical energy, thereby propelling the cars. The 

 electric motor " includes all apparatus through whose 

 employment work is performed." Everyone, no doubt, has 



