550 



Dr. John Hopkinson 



[April 26, 



a smaller magnet, 4 inches instead of 12 inches diameter, which 

 will get through the business nine times as speedily. I will 

 first of all throw upon the screen a diagram (Fig. 13) which shows 

 the method of the experiment. Here is the source of electricity, 

 marked " dynamo " ; here are resistances for regulating the current, 

 marked " rheostat " ; and here is a liquid reverser intended to 

 reverse the direction of the current by continuous steps. It consists 

 of two copper plates, to which the current is taken, and of two 

 moving plates revolving between these plates, which plates are 



Primary Connections. 



To D'Arsonval 

 Galvanometer. 



Fig. 13. 



connected to the electromagnet. As the moving plates revolve, the 

 current will diminish from a maximum in one direction to nothing, 

 and will then increase to a maximum in the opposite direction. The 

 electromagnet is marked " magnet," the secondary coils of which are 

 not indicated in the diagram. Only one coil in the centre of the 

 magnet is connected. We will now turn the reverser faster. You 

 see, the variations of inductions diminish greatly — indeed, the in- 

 duction at the centre of the core is but little affected by the changing 

 current in the magnetising coils (change leads). We might have 

 tried three coils instead of one, but the experiment is a little 



