THE THEORY or THE DYNAMO 233 



tendency to go in a particular direction, that it was a sort of gun shoot- 

 ing the lines of force through the armature. That is not true, because 

 they do not have any tendency to go that way at all, and we would only 

 add that much to the area of the end of the magnet. Very few lines of 

 force will go out there, and by putting this additional magnet on we 

 add to the area of the magnet. The lines of force will go out at the 

 sides probably in greater numbers than they would at the end, so that 

 I do not think that particular objection holds in that particular case. 

 It is a question of quantity; the thing should be measured and found 

 out. I see very plainly in my own mind that more lines of force would 

 go out the side by adding this iron here (Fig. 2) than would go out at 

 the end of it by leaving it vacant, as in Fig. 1. But it is a matter of 

 mere opinion. Another reason for having fewer magnets is that the 

 surface is greater in the case of the larger number than of the smaller 

 number for the lines of force to escape from. 



There was another point brought up here with respect to the machine 

 which was made in this way (Fig. 4). It was stated that there was 

 some gain from the magnetic action of this coil on the iron outside. 

 There is undoubtedly a gain: the question is how much, and whether 

 more lines do not escape than would make up for that. With no 

 experiments to go on, it is a case of judgment. My own judgment 

 would be that there would be very little gain ; but, as I said before, the 

 thing should be measured, and then we could find out about that point. 



[Discussion by Professors Sylvanus Thompson and Anthony and 

 others.] 



I am very glad that that point of hollow magnets has been brought 

 up, as I think that the question of hollow magnets, hollow lightning 

 rods, and a great many similar things, causes more difficulty, especially 

 to practical men, than almost anything else. It can be explained in 

 a very few words. Take a hollow bar having the magnetizing coil 

 around it acting to send lines of force along it. They have got to go 

 out to make their complete circuit. They could only end at a certain 

 point if we had free magnetism, that is, a separate magnetic fluid. 

 I speak not from a physical sense but from a mathematical point of 

 view. The principal resistance to the propagation of these lines of 

 force is in the air and not in the magnet. If we take away a large 

 portion of the interior of that magnet we will have the surface the 

 same as it was before, and consequently the external resistances are the 



