BY MONTAGUE RHYS JONES, C.E. 75 



and if an apology be needed for entering so fully into the 

 historical detail connected with the subject, that is my sole 

 excuse. 



Before proceeding to describe the methods of application, 

 and so as to afford those who have not had an opportunity of 

 studying the subject, it is proposed to briefly enter into an 

 historical and physical description of the dynamo and motor, 

 two of the most important agents in the application of 

 electricity as a motive power, which will, it is hoped, enable 

 them to have a thorough grasp of the principle. 



Before explaining the physical theory of the dynamo, it 

 may not be outside the scope of this paper to briefly refer to 

 Faraday's discovery in 1831 of the laws of magnetic-electric 

 induction. He found that induced currents could be started 

 or stopped in an adjacent coil, which led to the further dis- 

 covery that currents could be generated in a coil moved in 

 the poles of a powerful steel magnet. He then made what 

 he termed a. " new electrical machine," whose parts consisted 

 of a copper disc 12 diameter, and one-fifth of an inch in 

 thickness, fixed on a brass axle, and mounted in frames so as 

 to revolve, its edge being placed between the poles of a large 

 compound permanent magnet. Copper and lead conducting 

 strips were placed in contact with the edge of the disc, a wire 

 from the galvanometer was connected to the collecting strip 

 and the other to the brass axle. On revolving the disc a 

 deflection of the galvanometer was obtained, which was 

 reversed in direction when the direction of the rotation was 

 reversed. Here, therefore, was demonstrated the production 

 of a permanent current of electricity by ordinary magnets. 

 From 1832 many inventions and improvements followed, until 

 1848, when Brett made the important suggestion of causing 

 the current developed in the armature by the permanent 

 magnetism of the field magnets to be transmitted through a 

 coil of wire surrounding the magnet, so as to increase its 

 action. This improvement marked an era in the evolution of 

 the dynamo, being the first suggestion of the self-exciting 

 dynamo. Then followed a period of great activity, when 

 further improvements were effected, and in 1867 Dr. Werner 

 Siemens described in the Berlin Academy a machine for 

 generating electric currents by the application of mechanical 

 power, the currents being induced in the coils of a rotating 

 armature, by the action of electro-magnets, which were them- 

 selves excited by the currents so generated. To mark the 

 importance of this departure, Siemens coined the word 

 " dynamo-electric machine," which now is shortened into 

 " dynamo," and which has now become the name for all these 

 electric machines driven by mechanical power, whether self- 

 excited or not. Brush introduced his famous dynamo of a 



