224: HENKY A. KOWLAND 



attached somewhere. Very often the magnet is turned vertically, poles 

 downward, and attached to a cast-iron bench. I have no doubt that 

 some lines of force are lost (not much perhaps) in passing across from 

 the magnet to this iron bench. The makers of the machine, I suppose, 

 considered this to some extent, but what is needed is measurement on 

 that point. 



Here is another form of magnet (Fig. 5). That machine would be 

 defective. It has two magnets and two magnetic circuits in the place 

 of one, and many of the lines of force probably make little private cir- 

 cuits of their own around in that way (arrows). Those lines of force 

 are of course lost, and it is more or less defective in that respect. It 

 would be better to diminish the number of magnetic circuits to one. 



(I am only giving a general idea of the principle of these machines, 

 and I do not refer to any in particular.) 



It is also important that these lines of magnetic induction shall find 

 easy passage around in order to produce the most intense field. Thus 

 the opening between the armature and pole pieces must be made as 

 small as possible, in order that the lines of force may find easy passage 

 across it. Everybody recognizes that. Suppose we had a machine made 

 in the following manner (Fig. 6), in which there is a magnet with 

 a Gramme ring here (a), and pole piece here (&), a ring here (c), and 

 pole piece here (d), but no pole pieces opposite these. How are the 

 lines of force to pass around ? I do not know that it would be easy to 

 see how. They evidently go around here (arrows) and get to the other 

 side the best way they can. There is no easy passage around for the 

 lines of force in this case. 



A MEMBER. May they not to some extent follow the shaft ? 



Professor EOWLAND. It is evident that if the shaft is made large 

 enough some go along the shaft in that way (arrows), but there is no 

 easy way for them to get around. 



I have here a formula for the amount of work which one has to 

 expend upon a magnet in order to produce a certain effect. I will take 

 the case which I have considered most efficient, where there is one 

 magnetic circuit. It is an original idea of Faraday that these lines of 

 force are conducted. We suppose the lines of force to pass through 

 the iron and across the opening in this way (arrows, Fig. 1), and they 

 are caused to do that by what may be called the magneto-motive force 

 of the helix. 



I will just obtain an expression for the number of lines of force B. 

 This is not the quantity which Maxwell considers, but it includes the 



