1881.] Molecular Electro-Magnetic Induction. 



527 



port, the wire being 22 centims. long, having 2 centims. projection 

 beyond its support, in order to fasten upon it a key or arm which 

 shall serve as a pointer upon a circle giving the degrees of torsion 

 which the wire receives from turning this pointer. A binding screw 

 allows us to fasten the pointer at any degree, and thus preserve the 

 required stress the time required. 



The exterior diameter of the coil is 5-^ centims., having an interior 

 vacant circular space of 3 J centims., its width is 2 centims. ; upon 

 this is wound 200 metres of No. 32 silk-covered copper wire. This 

 coil is fastened to a small board so arranged that it can be turned 

 through any desired angle in relation to the iron wire which passes 

 through its centre, and it can also be moved to any portion of the 

 .20 centims. of wire, in order that different portions of the same wire 

 may be tested for a similar stress. 



The whole of this instrument, as far as possible, should be con- 

 structed of wood, in order to avoid, as far as possible, all disturbing 

 inductive influences of the coil upon them. 



The iron wire at its fixed end is joined or makes contact with a 

 copper wire, which returns to the front part of the dial under its board 

 and parallel to its coil, thus forming a loop, the free end of the iron 

 wire is joined to one pole of the battery, the copper wire under the 

 board is joined to the rheotome and thence to the battery. 



The coil is joined to the telephone ; but, as in every instance we can 

 either pass the battery current through the wire, listening to its induc- 

 tive effects upon the wire, or the reverse of this ; I prefer, generally, in 

 order to have no voltaic current passing through the wire, to join the 

 iron wire and its loop direct to the telephone, passing the voltaic 

 current through the coil. 



In order to balance, measure, and know the direction of the new 

 induction currents by means of a switching key, the sonometer* I 

 described to the Royal Society is brought into the circuit, The two 

 exterior coils of the sonometer are then in the circuit of the battery, 

 and of the coil upon the board containing the iron wire or stress 

 bridge. The interior or movable coil of the sonometer is then in the 

 circuit of the iron wire and telephone. Instead of the sonometer con- 

 structed as described in my paper to the Royal Society, I prefer to 

 use one formed upon a principle I described in " Comptes Rendus," 

 December 30, 1878. This consists of two coils only, one of which is 

 smaller and turns freely in the centre of the outside coil. The exterior 

 coil being stationary, the centre coil turns upon an axle by means of a 

 long (20 centims.) arm or pointer, the point of which moves over a 

 graduated arc or circle. Whenever the axis of the interior coil is 

 perpendicular to the exterior coil, no induction takes place, and we 



* " Proc. Eoy. Soc," vol. 29, p. 65. 



