230 SCIENTIFIC RECREATIONS. 



by filing the edge all round. If instead of the plaster cast a copper coin or 

 other copper object be used, the blackleading is not required, but the 

 surface must be first made clean and bright. 



Many uses are made of the galvanic current by medical men. If the 

 circuit of the pile is closed and we take a wire in each hand and break 

 contact, a concussion will be felt in the joints of the arm and fingers, and a 

 certain contraction of the muscles. The currents of electricity cause the 

 shocks, and by a peculiar arrangement by which the circuit can be closed or 

 broken at pleasure, a series of shocks can be sent through the body when it 

 forms the connection between the poles of the battery. We give illustra- 

 tions of a medico-galvanic machine. In fig. 235 there are two batteries, A and 

 B, with cells, C D. Each battery consists of a central plate of platinized 

 silver separated from the zinc plates by a piece of wood, E and F ; the 

 binding-screws are fastened to the silver plates, and G H retain the zinc plates; 

 I is a copper band connecting the zinc plate of one battery with the silver 

 plate of the other. At z and opposite are wires leading to the coil machine. 

 The quantity and intensity of the current are regulated respectively by the 



Fig. 236 Fig. 237. Fig. 238 



Horse-shoe magnets, 



indicator, o, and the wires, Q. There is a point, RS, for the breaking of the 

 contact ; P N are screws retaining the wires which lead to the handles, uv, 

 grasped by the patients. 



The electric current is employed in many diseases, and is of great use 

 in some cases, but the further consideration of it with reference to its medical 

 applications does not fall within the scope of our present work. We will 

 uow pass on to one of the most useful applications of the electric force, the 

 Telegraph, and in dealing with it we must make a few remarks upon magnetism. 

 First, let us make an experiment or two, and see the reciprocal action between 

 electricity and magnetism. 



(i.) If we take a piece of iron of the form of a horse-shoe (fig. 236), and 

 wind around it copper wire, and pass through the wire an electric current from 

 our battery, the iron will exhibit strong magnetic properties, which it will lose 

 when the current is interrupted. The conducting wires are insulated with silk, 

 and the current will then travel in one direction. 



(2.) If we cover the ends of a non-magnetic piece of iron with coils 

 of wire, and rotate the magnet, A B, so as to cause the poles to approach 



