demonstrating a limit to the Magnetiz ability of Iron. 309 



^^' Nothing can be more striking than the difference between the 

 ratios of lifting to distant attractive power_, in the different mag- 

 nets ; whilst the steel magnet attracts with a force of 23 grains 

 and lifts 60 oz., No. 3 attracts 5-1 grains and lifts 92 oz. 



The following are some general directions for making electro- 

 magnets for lifting : — 1st. The magnet, if of considerable bulk, 

 should be compound, and the iron used of good quality and well 

 annealed. 2nd. The bulk of the iron should bear a much 

 greater ratio to its length than is generally the case. 3rd. The 

 poles should be ground quite true, and fit flatly and accurately 

 to the armature. And 4th. The armature should be equal in 

 thickness to the iron of the magnet. 



I shall now proceed to consider with greater care what form 

 of electro-magnet is best for distant attraction, as that is the 

 only force of any use in the electro-magnetic engine. Here two 

 things must be considered ; the length of the iron, and its sec- 

 tional area. 



Now with regard to the length of the iron, I have found that 

 its increase is always accompanied with disadvantage, unless the 

 wire is (by using a shorter length) forced to too great a distance 

 from the iron. In making magnets for an engine, it will be 

 proper to use a length less than that which gives the maximum 

 of attraction, on several accounts. 



The next thing to be considered is the sectional area. You 

 have shown* that, on placing a hollow and solid cylinder of iron 

 successively within the same electro -magnetic coil, the hollow 

 piece exerted the greatest influence on the needle. I wished to 

 ascertain whether a hollow magnet could be represented by a 

 solid one, of which the sectional area and circumference are the 

 same, and the thickness of which is twice that of the hollow 

 magnet. The accompanying figures represent 

 sections of hollow and solid rectangular mag- 

 nets ; and it will be seen, that if either of them 

 is divided at the dotted lines, the separate pieces, 

 when put properly together, will make up the 

 other. Two electro-magnets were constructed, 

 each 7 inches long, and wound with twenty-two feet of insulated 

 copper wire ; the sections were similar to, but twice the size of the 

 figures. Their attractions at half an inch distance for the con- 

 trary pole of a straight steel magnet were as follow : — 



Hollow magnet. Solid magnet. 



Attraction in grains 1-9 1*7, 



Do. with a more powerful battery . 4*5 4*0 ' 



The above results show that the hollow magnet has the greater 

 * Annals of Electricity, vol. i. p. 470. 



