1879.] 



On an Induction- Currents Balance. 



57 



In order to have a perfect induction- currents balance suitable for 

 physical research, all its coils as well as the size and amount of wire 

 should be equal. The primary and secondary coils should be separate 

 and not superposed. The exterior diameter of the coils presented this 

 evening is 5^ centims., having an interior vacant circular space of 

 3^ centims. ; the depth of this flat coil or spool is 7 millims. 



Upon this box-wood spool are wound 100 metres of No. 32 silk- 

 covered copper wire. I use four of such coils, formed into two pairs, the 

 secondary coil being fixed permanently, or by means of an adjustable 

 slide, at a distance of 5 millims. from its primary; on the second 

 similar pair there is a fine micrometer screw, allowing me to adjust 

 the balance to the degree of perfection required. 



These two pair of coils should be placed at a distance not less than 

 1 metre from each other, so that no disturbing cause should exist from 

 their proximity. 



The two primary coils are joined in series to the battery, the circuit 

 also passing through the microphone. 



In place of the telephone I have sometimes used a magnetic 

 pendulum, the swing of the arc described indicating and measuring 

 the forces.* I am at present engaged upon a very sensitive 

 voltameter which shall indicate and measure the force of rapid in- 

 duced currents. The telephone, however, is well adapted as an 

 indicator, but not as a measurer of the forces brought into aotion. 

 For this reason I have joined to this instrument an instrument to 

 wnieh I have given the name of electric sonometer. This consists of 

 three coils similar to those already described, two of which are placed 

 horizontally at a fixed distance of 40 centims. apart, and the com- 

 munication with the battery is so arranged that there are similar but 

 opposing poles in each coil ; between these there is a coil which can be 

 moved on a marked sliding scale divided into millimetres ; in a line 

 with these two opposing primary coils, the centre coil is the secondary 

 one, and connected by means of a circuit changing key with the 

 telephone in place of the induction balance. If this secondary coil is 

 near either primary coil, we hear loud tones, due to its proximity. 

 The same effect takes place if the secondary coil is near the opposing 

 coil, except that the induced current is now in a contrary direction, as 

 a similar pole of the primary acts now on the opposite side of the 

 induction coil ; the consequence is, that as we withdraw it from one 

 coil approaching the other, we must pass a line of absolute zero, where 

 no current whatever can be induced, owing to the absolutely equal 

 forces acting equally on both sides of the induction coil. This point 

 is in the exact centre between the two coils, no matter how near or 

 distant they may be. We thus possess a sonometer having an abso- 

 lute zero of sound ; each degree that it is moved is accompanied by 

 * "Telegraphic Journal," December 15, 1878. 



