228 Lord Rayleigh on the Behaviour of Iron and Steel 



C is here expressed in C.G.S. measure, on which scale the 

 ampere is *1. 



It may be objected that the magnetic force of the spiral is 

 not the only external force operative upon the iron. It is 

 true that the compensating-coils must have an influence, and 

 in the opposite direction. But calculation shows that the 

 influence must be small. The radius of the large coil is 

 9 centim., and (to take an example) the distance of its mean 

 plane from the suspended needle in one set of experiments on 

 hard iron was 13*6 centim. Under these circumstances the 

 magnetic force in the spiral, even at the nearer end, is influ- 

 enced less than 2 per cent, by the large compensating-coil. 

 The effect of the smaller coil is about the same. For the 

 present purpose it is hardly worth while to take these correc- 

 tions into account. 



As has been remarked, the coils of the apparatus were 

 always connected in series ; but a reversing-key (serving also 

 to make and break) was introduced so as to allow of the re- 

 versal of the compensating-coil in relation to the others. In 

 one position of the key ( — ) the action of the coil and of the 

 magnetized iron are opposed ; in the other ( 4- ) the actions 

 conspire. When the currents to be used were not exceedingly 

 small, the whole apparatus was in simple circuit with a Daniell 

 cell and such resistance-coils as were necessary. Exclusive 

 of the cell and of the added resistances, the whole resistance 

 was 1\ ohms. 



As an example, I will now give the details of some obser- 

 vations on December 6 made to test the behaviour of unan- 

 nealed Swedish iron wire. The diameter of the wire is 

 1*6 millim. ; it is from the same hank as a piece used in the 

 experiments of Part II.* The compensating-coil was adjusted 

 until it made no difference whether the key was open or 

 closed ( — ), the additional resistance being 1000 ohms. In 

 stating the result it will for the present be sufficient to give 

 the German-silver resistances, that of the apparatus and of the 

 battery being relatively of no importance. The corresponding 

 current is about 10~ 4 C.G.S. , and the strength of the magnetic 

 field in the spiral is given by 



128ttC = -04 C.G.S. 



We shall have a better idea of this if we recall that, on the 

 same system of measurement, 



H = '18; 



so that the force in action is about \ of that which the earth 

 exercises horizontally. 



* L. e. p. 488. 



