1835.] BecquereVs Traite Experimental, Sfc. 149 



atmosphere is always proportional to the electrical density ; and it 

 follows, from the experiments of Lord Stanhope, that the density of 

 the electricity of electrical atmospheres diminishes inversely as the 

 square of the distance from the excited body. But, as the demon- 

 stration of Coulomb's law has been very clearly stated by Dr. Thom- 

 son in his work on electricity, which, as it contains a full and accurate 

 description of electrical apparatus, and of most subjects contained in 

 the remainder of Becquerel's book relating to electricity, we shall beg 

 leave to refer to, as being more easily understood by the English 

 reader, and proceed to the section on 



II. Magnetism. — After treating of the general properties of 

 magnetic needles, Becquerel proceeds to consider the earth's action 

 upon magnets. He describes very particularly Pouillet's compass 

 for determining the declination, and then brings forward some of the 

 results obtained by its means. The determination of the resultant 

 of the magnetic force of the globe is a point of importance, which is 

 measured by the number of oscillations of the needle occurring in a 

 given time, the needle acting in relation to the terrestrial magnetic 

 force, as the pendulum in connexion with gravity. Coulomb has 

 proved that in reference to the magnetic momentum, the number 

 19 17*76 represents the force of torsion, which would be necessary to 

 maintain the magnetic needle at 90°. The forces of torsion for the 

 deviation of a small number of degrees being proportional to the 

 angles or their signs, we have 

 force of torsion = mj fi ^ sin fa j = then force rf 



sin. 90° 

 torsion 19l7"76. He found also that the momentum of needles is 

 nearly as the squares of the length of the threads of suspension. 



2. The force of magnetic attraction and repulsion is inversely as 

 the square of the distance. 



3. The author then proceeds to explain the method of forming 

 magnets, and describes particularly the application of Gilbert's dis- 

 covery by Scoresby, by which the magnetic influence can be imparted 

 to bars of iron without the aid of magnetized bodies. Two bars of 

 steel are taken, 30 inches in length, with two other plate bars of 

 steel 8 inches long and half an inch broad, and a long bar of iron, 

 all of them destitute of magnetic power. 



The large bar of iron is first struck in a vertical position, and then 

 placed, without changing its direction, upon the steel bars which 

 have also been struck. They are then struck upon each other. 

 Each of the small bars, suspended vertically to the summit one of 

 the large steel bars, is successively struck, and in a few minutes they 

 acquire a considerable magnetic power. Two more of the small bars 

 united by two small iron parallepepids are rubbed with the four bars, 

 and are then replaced by two others, and these again by the two last. 

 Each pair of bars being then treated for a certain space, and being 

 changed after exposure to friction for a minute, is found completely 

 saturated with magnetic fluid. This method of preparing magnets 

 is not only more simple than those of the double touch of Mitchell 

 and Canton, and of successive contact of Duhamel and Alpinus, but 

 it increases considerably the magnetic intensity of the bar. 



