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XVIII. Experiments on the Directive Poiver of large Steel Magnets , of Bars of magnetized 
Soft Iron, and of Galvanic Coils, in their Action on external small Magnets. By 
George Biddell Airy, Astronomer Boyal, C.B., P.B.S . — With Appendix, containing 
an Investigation of the Attraction of a Galvanic Coil on a small Magnetic Mass. 
By James Stuart, Esq., M.A., Fellow of Trinity College, Cambridge. 
Received January 6, — Read February 8, 1872. 
The only experiments with which I am acquainted tending to throw light upon the 
distribution of magnetic power in the different parts of a steel magnet are some very 
imperfect ones by Coulomb in the French Memoirs for 1789 and other years. It 
appeared to me that it might be desirable to make experiments of a rather more exten- 
sive character, and to add some measures of the magnetic effect of galvanic currents, 
both directly by their immediate action, and indirectly by the amount of magnetic power 
which they produce inductively in soft iron. 
For the measure of permanent magnetism I selected a bar magnet 14 inches long, 
1*4 inch broad, 0'35 inch thick; it has not been touched by a magnet for several years, 
and is likely to be in a state of very permanent magnetism. For the galvanic currents 
a cylindrical coil was used 13-4 inches long, 1*4 inch in external diameter, and about 
09 inch in internal diameter; it has, I believe, four layers of wire, each layer having 
160 revolutions of the wire. The battery used with it consisted of three cells, with 
sulphuric acid diluted in the proportion of 1 to 6 ; the plates were of zinc and graphite, 
each exposing on each side about 8 square inches ; the circuit was always completed 
about half an hour before the experiments were begun, and a delicate galvanometer was 
placed in circuit by which the steadiness of the current was established. A core of iron 
0-8 inch in diameter and of the same length as the coil, removable at pleasure, fits well 
in the inside of the coil ; the iron is quite soft, and can with ease be entirely freed from 
any subpermanent magnetism. 
The first step in the experiment was to neutralize terrestrial magnetism within the 
area of magnetic experiment. For this purpose two powerful 2-feet magnets were 
placed below the table on which the experiments were made, with their red or north- 
seeking ends directed to the magnetic north, at a distance (determined by trial) such 
that the experimental compass was sensibly uninfluenced by terrestrial magnetism. It is 
possible that some small residual magnetism was perceptible in the comparison with the 
feeble galvanic action, but none could be certainly discovered in the other experiments. 
The compass used for register of the magnetic action is a small and very lively pocket- 
compass, with needle 1*0 inch long, not loaded with a card. The box of this compass 
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