50 Mr. Shelford Bidicell [Feb. 21, 



WEEKLY EVENING MEETING, 



Friday, February 21, 1890. 



John Eae, M.D. LL.D. F.E.S. Vice-President, in the Chair. 



Shelford Bidwell, Esq. M.A. LL.B. F.R.S. M.B.L 



Magnetic Phenomena. 



The space around a magnet in which magnetic action is observed is 

 called a field of magnetic force, or, more shortly, a magnetic field. 

 Following Faraday's conception, we may specify a magnetic field by 

 supposing it to be filled with a number of so-called " lines of force," 

 the direction of the force (that along which a north pole is urged) 

 beinfy indicated by the direction of the lines, and its intensity by 

 their concentration. In a uniform field of unit intensity, the lines 

 of force are straight and parallel, and each line is exactly one 

 centimetre distant from its nearest neighbour ; so that, if a flat surface 

 were held transversely to the direction of the lines, one line would 

 pass through each square centimetre of the surface. In a weaker 

 field the lines would be farther apart ; in a stronger one they would 

 be packed more closely together. The direction of the earth's 

 magnetic force at any point in or near London is, roughly speaking, 

 from south to north, at an inclination of 67° to the horizon; its 

 intensity is approximately such that one line of force traverses every 

 two square centimetres of a transverse plane surface, i. e. half a line, 

 for each unit of area. The intensity of a unit field of magnetic force 

 is therefore equal to about twice the total intensity of the magnetic 

 field due to the earth. 



It is a remarkable fact that iron, and in a less degree the two 

 rarer metals nickel and cobalt, when placed in a magnetic field, 

 possess the property of multiplying the number of lines that would 

 naturally fill the space occupied by them. Thus, a long and thin iron 

 rod placed lengthwise in the earth's magnetic field will not merely 

 be traversed by half a line for each square centimetre of its section, 

 as a glass or copper rod would be ; the half line will (at least in the 

 middle portion of the rod) be multiplied something like 600 times, 

 raising the actual number of lines through the iron to about 300 per 

 centimetre of section. 



By means of electric currents it is easy to produce magnetic 

 fields having a far higher intensity than that of the earth. Suppose, 

 for example, we take a long brass tube, and wind around it a quantity 

 of insulated copper wire, forming 16 convolutions in each centimetre 

 of length ; a current of 10 amperes circulating through such a coil 

 would generate in the interior a magnetic field having an intensity of 

 about 200 units. An iron rod placed inside this tube would be 



