310 



Mr. J. P. Joule's Account of Experiments 



attractive force ; but I do not think that the difference between 

 the two ia so great as to counterbalance the practical advantages 

 which solid bars would give if used in the engine. I shall now 

 therefore attempt to determine the sectional area of solid iron 

 most proper for various galvanic powers. 



I made five straight electro-magnets of square iron wire y^th 

 of an inch thick ; each was 7 inches long, and wound with 

 twenty-two feet of insulated copper wire of y^^rth of an inch dia- 

 meter. No. 1 consisted of nine, No. 2 of sixteen, No. 3 of 

 twenty-five, No. 4 of thirty-six, and No. 5 of forty-nine square 

 iron wires, arranged in the form of square prisms. Five other 

 electro-magnets were made of square iron rod, but in eveiy other 

 respect were exactly similar to the first. The following are the 

 attractions (at half an inch distance) for a straight steel magnet, 

 with three different voltaic forces. 



1st experiment. 



Attraction of iron bar 1 

 magnet in grains.../ 



tttn n( wiro mnomof 



iliagltct 111 giaiuo... 



Ditto of wire magnet 



^ , . . r Iron bar magnet 



2ndexpenraent.|^i^g magnet 



q~i *^««^«,«„* / '""on liar magnet 



3rd experiment. I ^y.^g magne? 



1-5 



31 

 20 

 2-6 

 27 

 3-3 



No. 2. 



1-9 



21 



2-5 

 2-8 

 36 

 3-8 



No. 3. 



1-6 



17 



2-35 



21 



3-4 



30 



No. 4. 



21 



20 



245 



22 



3-2 



2-9 



No. 5. 



2-0 



1-9 



22 



205 



31 



2-65 



The square iron wire of which the wire magnets were con- 

 istructed, was taken at the same degree of temper that it pos- 

 sessed when it came from the manufacturer. It was in con- 

 sequence not so well annealed as the iron bars. On this account 

 the numbers opposite the wire magnets are less than they would 

 have been with better annealed wire : still the results of the table 

 seem anomalous ; for it will be remarked, that whilst the wire 

 magnets are the most powerful of the smaller electro-magnets, 

 the bar magnets are most powerful of the larger ones. 



* * * 



I remain^ &c.. 



J. P. Joule, 



Investigations in Magnetism and Electro-Magnetism. 

 By James P. Joule. In two Letters to the late Mr. Sturgeon*. 



Broom Hill, near Manchester, 

 Dear Sir, May 28, 1839. 



I am now able to send jrou an account of my further investi- 

 gations on electro-magnetic attraction. It was judged to be a 

 matter of the first importance, in a research like the following, 



♦ Axmula of Electricity, vol, iv. p. 131, 



