536 Mr. R. H. M. Bosanquet on Electromagnets. 



The sketch represents the instrument constructed for these 

 experiments. This consists of two cylindrical iron magnets, 

 each 20 centim. long and *526 centim. in diameter. Each is 

 wound with 1096 coils of wire. One magnet is fixed in a 

 wooden frame, while the other moves exactly under it smoothly 

 and without friction, between two brass collars (not shown), 

 through a vertical distance of about a quarter of an inch. 

 The magnets were at first ground together so as to ensure a 

 perfect contact. The lower or sliding magnet has attached to 

 its lower end an iron hook, which passes through the table 

 and sustains the wooden pan for the reception of the weights. 

 To this magnet also is firmly attached a brass collar, in which 

 are two holes for the pivots of the beam. The beam which 

 carries the counterpoise is about 18 inches long ; it moves on 

 knife-edges, and has at one end a half-round fork, which em- 

 braces freely the collar on the lower magnet, and is pivoted 

 thereto. On the other end of the beam is hung a lead coun- 

 terpoise exactly balancing the wooden pan and magnet. 



The magnetizing current was sent through both electro- 

 magnets, while the induction was measured* by small coils 

 placed round the point of contact of the two magnets. The 

 inductions were measured by reversal. Weights were care- 

 fully added until the contact was broken, and the greatest 

 weight sustained was taken as the observed number. This 

 could be observed with certainty to the nearest ounce. 



The first result is that the formula, 



SIS 2 

 (Wi) weight in grammes = -~ ^j (S = section of contact), 



derived from (vol. ii.) Clerk Maxwell's * Electricity and Mag- 

 netism,' p. 256, represents the values well on the whole ; but 

 for very small inductions the w T eight sustained is several times 

 as great as it should be according to this theory. And this did 

 not arise from residual induction ; for when the magnetizing 

 current was interrupted, no power was left to sustain any 

 weight at all. 



I therefore framed the empirical formula W , involving 

 both % and % 2 ; this represents the lower and middle values 

 fairly, but not the highest. 



It cannot, however, escape notice that the observed values 

 are best satisfied by adding about 4 oz. to all weights calcu- 

 lated from "Wj. Since, however, this addition must vanish 

 for M = 0, we cannot thus obtain a general formula. Note 



* For the method see Phil. Mag. ser. 5, xix. p. 75. 



