196 Mr. 0. Heaviside on the 



early experiments I was sufficiently satisfied by finding 

 that the substitution of a bundle of iron wires for a solid 

 iron core, with a continuous reduction in the diameter 

 of the wires, reduced the diffusion effect to something 

 quite insignificant in comparison with the effect when the 

 core was solid, to conclude that we had only to stop the 

 flow of currents to make iron, under weak magnetizing 

 forces, behave merely as an inductor. More recently, on 

 account of some remarks of Prof. Ewing on the nature 

 of the curve of induction under weak forces, I immensely 

 improved the test by making and using nonconducting cores, 

 containing as much iron as a bundle of round wires of the 

 same diameter as the cores. I take the finest iron filings (sitt- 

 ings) and mix them with a black wax in the proportion of 1 of 

 wax to 5 or 6 of iron filings by bulk. After careful mixture 

 I roll the resulting compound, when in a slightly yielding 

 state, under considerable pressure, into the form of solid round 

 cylinders, somewhat resembling pieces of black poker in 

 appearance. (-J inch diameter, 4 to 6 inches long.) That 

 the diffusion effect was quite gone was my first conclusion. 

 Next, that there was a slight effect, though of doubtful 

 amount and character. The resistance-balance had to be very 

 carefully attended to. But, more recently, by using coils 

 containing a much greater number of windings, and thereby 

 increasing the sensitiveness considerably, as well as the 

 magnetizing force, I find there is a distinct effect of the 

 kind required. Though small, it is much greater than could 

 be detected ; but whether it should be ascribed to the cause 

 mentioned or to other causes, as dissipation of energy due to 

 variations in the intrinsic magnetization, or to slight curvature 

 in the line of induction, so far as the quasi-elastic induction is 

 concerned, is quite debateable. To show it, let 1 and 2 be 

 equal coils wound together (L = 3 macs, R=47 ohms), 3 and 

 4 equal in resistance (R 3 = R 4 =93 ohms), but of very unequal 

 inductances, that of coil 3 (L 3 = 24 macs) being so much 

 greater than that of coil 4 that the iron core must be fnlly 

 inserted in the latter to make L 4 = L 3 . (Coils 3 and 4; 1\ inch 

 external, \ inch internal diameter, and f inch in depth. Fre- 

 quency 500.) The balancing of induction is completed by 

 means of an external core. Resistance of branch 6 a few 

 ohms, E.M.F. 6 volts. There is, of course, immense sound 

 when the core is out of coil 3, but when it is in there is merely 

 a faint residual sound which is nearly destroyed by increasing 

 R 3 by about q^q part, a relatively considerable change. On 

 the other hand, pure self-induction of copper wires gives 

 perfect silence, and so does M 64 , a method I have shown to be 



