I70 



NA TURE 



[June 13, 1901 



causing the magnetism passing through the secondary circuit 

 connected to the ballistic galvanometer to rapidly die out, and 

 in doing so a quantity of electricity, proportional to the aniount 

 of magnetism, is sent through the galvanometer, thus giving a 

 measure of the amount of magnetism. Fig. 6 shows the 

 difference between three specimens of iron— I. cast iron, II. 

 wrought iron. III. best cast steel for magnets. 



The curves show how rapidly the magnetic resistance of iron 

 rises as the density of magnetisation is increased, and therefore 

 the importance of not allowing the density to exceed about 



'• Ampere turns per 



i6,ooo C.G.S. units. These curves are very useful in design- 

 ing, because from them the ampere turns necessary to produce 

 a certain density of magnetisation in a particular kind of iron 

 can at once be found. It is most important to use the best 

 steel, such as curve III. represents, for making magnets that 

 are to produce very strong fields. With regard to what is 

 meant by a very strong field, a field up to 20,000 C.G.S. 

 units is moderately easily reached. But at about this limit 

 saturation of the iron sets in and it becomes much more 



difficult to go higher. A field of 30,000 units is a very strong 

 field ; the cost of the magnet rises very rapidly if a stronger field 

 than this is required. A field of 40,000 is about the strongest 

 field obtainable. To go above this would require such a large 

 additional expenditure in materials and power, compared with 

 the small increase in the field, that it is not practical. 



Fig. 7 shows a photograph of a good type of magnet to 

 produce a very intense field. In this magnet the density 



NO. 1650, VOL. 64] 



of flow in the pole pieces is very great, so great, 

 in fact, that the permeability of the pole pieces is not 

 much greater than that of air ; consequently a very large 

 leakage of magnetism occurs, which makes the calculations 

 very laborious. An idea of the amount of leakage that 

 occurs is got from the fact that the flow of magnetism ii> 

 the lower cylinder. Fig. 7, is fifty times greater than the flow 

 across the air gap under working conditions, whereas in 

 the case of the magnets of dynamo machines the flow in the 

 yoke would not generally be more than i -4 times the flow in 

 the air gap, because they produce only relatively weak fields of 

 about 6000 units. Owing to this great leakage the calculations 

 cannot be made so accurately for this magnet. Fig. 7, as for 

 dynamo magnets. In making the final calculations for this 

 magnet it was necessary to divide the magnetic circuit in the 

 pole pieces up into a great many sections, only two millimetres 

 apart, finding the ampere turns necessary for each section, 

 because the density changes so rapidly in the pole pieces, and 

 also because the ampere turns required for the pole pieces are 

 much greater than those required for the air gap, which 

 cannot be helped, because the magnet was designed to produce 

 a very intense field in a small air gap. 



The following table gives the results of the final calcula- 

 tions : — 



For Magnet, Fig. 7. 



This magnet was originally intended to produce a field of 

 35,000 C.G.S. units, in an air-gap i-inch long, for which the 

 above calculations were made ; but they show that the magnet 

 will not be able to produce so strong a field as this, because it is 

 wound so that it may be connected straight on to 200-volt 

 mains, with all coils in series, or lOO-volt mains with two sets 

 of coils in parallel, and then there are 22,400 ampere turns 

 available, whereas at least 24,120 are required. 



Recently this magnet was made by the Electric Construction 

 Company for the Solar Thysics Laboratory, South Kensington, 

 and the strength of the field produced under the conditions 

 assumed in the above calculation was found to be 32,000 C.G.S. 

 units, showing that the theoretical calculations agree fairly well 

 with practice, considering the very high value of the field. 



When the length of the air-gap was reduced to 2 millimetres, 

 and the exciting current doubled, the maximum strength of 

 field attained, for a short time, was nearly 38,000 C.G.S. units, 

 which corresponds to a pull, between the pole pieces, of no less 

 than S30 pounds per square inch ! T. L. James. 



