1879.] 



On an Induction- Currents Balance. 



59 



hearing power, we determine on a constant standard of force such as 

 one element Daniell, but if we increase the number of elements 

 we in same ratio increase the inductive disturbance, and thus by a 

 large increase of force bring within our range results too feeble to be 

 heard without its aid, the sonometer constantly, however, giving the 

 same degree for equality as the increased force is also used on this 

 instrument. Thus in our measurements we can entirely neglect the 

 amount of battery, as its comparative results remain a constant. 



As a rule three Daniell elements will be found quite sufficient, and 

 even this weak force is so exquisitely sensitive that it will find out 

 the smallest fraction of difference in weight or structure of metals. 

 Thus two silver coins such as a shilling, both quite new, and both 

 apparently of the same weight, will be found to possess a difference of 

 weight which the instrument at once indicates. 



The following experiments will show its exceeding sensitiveness 

 and its wide field of usefulness as an instrument of research. 



I. If we introduce into one pair of the induction coils any conduct- 

 ing body, such as silver, copper, iron, &c, there are set up in these 

 bodies electric currents which react both upon the primary and 

 secondary coils, producing extra currents , whose force will be pro- 

 portional to the mass, and to its specific conducting powers. A mili- 

 gramme of copper on a fine iron w T ire, finer than the human hair, can 

 be loudly heard and appreciated by direct measurement, and its exact 

 value ascertained. We can thus weigh to an almost infinitesimal 

 degree the mass of the metal under examination ; for instance, if we 

 take two English shilling pieces fresh from the Mint, and if they are 

 absolutely identical in form, weight, and material, they will be com- 

 pletely balanced by placing one each in the two separate coils, pro- 

 vided that for these experiments there is an adjustable resting place 

 in each pair of coils, so that each coin may lie exactly in the centre of 

 the vacant space between the primary and secondary coils. If, however, 

 these shillings are in the slightest degree worn, or have a different 

 temperature, we at once perceive this difference, and if desired, 

 measure it by the sonometer, or, by lifting the supposed heaviest coin 

 at a slight distance from the fixed centre line, the amount of degrees 

 that the heaviest coin is withdrawn will show its relative mass or 

 weight as compared with the lightest. I have thus been able to ap- 

 preciate the difference caused by simply rubbing the shilling between 

 the fingers, or the difference of temperature by simply breathing near 

 the coils, and in order to reduce this sensibility within reasonable 

 limits, I have only used in the following experiments 100 metres of 

 copper wire to each coil and 3 cells of battery. 



II. The comparative disturbing value of disks of different metals, 

 all of the same size and form of an English shilling, and measured in 

 millimetre degrees, by the sonometer, is the following : — 



