158 The Astronomer Royal's Magnetic Experiments. [Feb. 8, 



III. " Experiments on the directive power 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 Royal, C.B., P.R.S. — With Appendix con- 

 taining an Investigation of the Attraction of a Galvanic Coil on 

 a small magnetic mass. By James Stuart, Esq., M.A., Fel- 

 low of Trinity College, Cambridge. Received January 6, 1872. 



(Abstract.) 



The author, after adverting to some imperfect experiments made by 

 Coulomb in the last century, describes the apparatus which he had himself 

 used. He employed a bar-magnet 14 inches in length, placed in one series 

 with its edge towards the small compass on which its directive power was 

 estimated, and in another series with its flat side towards the small 

 compass ; also a galvanic coil 13*4 inches in length, animated by a battery 

 of three cells, and the same coil with the insertion of a soft iron coil. In 

 the field of experiment the earth's magnetism was sensibly neutralized by 

 external large magnets. The direction of the needle of the small compass 

 was estimated by eye. The magnitude of the directive force was found by 

 observing the position taken by the needle when the poles of a horseshoe- 

 magnet were placed in a definite position above it : for the measure of the 

 force of the galvanic coil without core, a very small magnet was used in 

 the same manner; its power was found to be about yj^ that of the 

 norseshoe-magnet. The circle on which the deflections were observed was 

 graduated to cotangents, which gave immediately the measure of the force 

 of the large magnet or coil, &c. In each case, observations were taken 

 in 30 stations in one oval ring surrounding the magnet &c, and in 38 

 stations in another oval ring surrounding it at a greater distance. 

 Omitting notice of the measures in general, the following specific points 

 are remarked : — 



At a constant distance from the steel, the greatest force exerted by a 

 magnet is not the longitudinal force at the end, but the transversal force 

 near the end. In going round the magnet there are six maxima and six 

 minima of force. 



The law of attraction of the core of a galvanic coil is not very different 

 from that of a magnet. 



The force produced by the core within the coil is very much greater 

 than that produced by the coil alone. In some positions of the small 

 compass it is about forty times as great, and in some about 1 70 times as 

 great. 



The law of force at different parts of the coil differs greatly from that at 

 corresponding parts of the magnet or core. In the coil it is, proportion- 

 ally, far greater at the end, and its direction is different. Near the end of 

 the magnet or core the directions of force converge to a point within it, 



