Lifting Power of the Electro-Magnet. 295 



to offer any mode of distinguishing between the forces M, D, and C. The first 

 may be perhaps less accurate as to individual measures, or at least requires 

 greater care and more numerous repetitions than the method of deflection ; but 

 these are probably more than compensated by the magnitude of the quantity to 

 be measured. In applying it I have examined — 



1. The relation between a magnet's power and the intensity of the current 

 passing through its helices. 



2. The effect of varying the number of spires in the helices and their dis- 

 tribution on the magnet. 



3. The change produced by varying the unexcited portion of the magnetic 

 circuit. 



4. The difference between electro-magnets of iron and those of steel ; and 



5. The influence of the length and diameter of the magnet. 



The first of these is the subject of the present communication, reserving the 

 others for another opportunity. 



The apparatus which I used in making these experiments consists of an 

 electro-magnet, a weighing apparatus, and the instruments for measuring and 

 regulating the exciting current, each of which requires some notice. 



1. The magnet consists of two cylinders of iron (the softest and most ho- 

 mogeneous that I have ever seen), each twelve inches long and two in diameter. 

 They were made hollow, as from Barlow's experiments I had imagined that 

 the central portion added little to the effect ; and I purposed to experiment at 

 temperatures above boiling water, by introducing heaters in these cavities. I 

 find that in this I was mistaken,* but the results are merely reduced in propor- 

 tion to the transverse section, or as 3 : 4, the cavity being one inch diameter. 

 The cylinders are screwed, with their axes 6 inches apart, into a base of the 

 same iron, 2 inches deep, and 2^ broad ; together they weigh 26 lbs. The 

 keeper is a rectangular prism, the same size as the base, weighing 7 lbs. It was 

 planed and fitted so carefully, by scraping, to the polar surfaces of the cylin- 

 ders, that it all but adheres to them by atmospheric pressure ; and was then 

 fitted with guides, so as always to insure uniformity of contact. 



* Mr. Bergin, with my helices on a solid magnet of the same dimensions, obtained with a 

 current = I'UllT, a lift of 670'8 lbs. This magnet, with the same current, gives 509'2 ; the num- 

 bers are as 4 : 3'03. 



