May 22, 1879] 



NATURE 



79 



difference of weight which the instrument at once indi- 

 cates. 



The following experiments will show its exceeding 

 sensitiveness and its wide field of usefulness as an instru- 

 ment of research. 



1. If we introduce into one pair of theinduction coils 

 any conducting 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, pro- 

 ducing extra currents whose force will be proportional to 

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

 gramme of copper on a fine iron wire, 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 completely balanced by placing one each in 

 the two separate coils, provided that for these experi- 

 ments there is an adjustable resting-place in each pair of 

 coils, so that each coil 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 sono- 

 meter, 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 degrees that the heaviest coin is withdrawn will show its 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 appreciate 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 ex- 

 periments 100 metres of copper wire to each coil and 

 three cells of battery. 



2. The comparative disturbing value of discs of different 

 metals, all of the same size and form of an English 

 shilling, and measured in millimetre degrees, by the sono- 

 meter, is the following : — 



These numbers do not agree entirely with any lists of 

 electrical conductivity I have yet met with ; the numbers 

 are, however, invariably given by the sonometer, and the 

 divergence may be due to some peculiarity of structure of 

 the metals when formed into disks. Future investigations 

 with this instrument will, no doubt, give more correct 

 values than I hare been able to obtain with my limited 

 means of research. 



3, It will be seen from the above that the instrument 

 gives very different values for different metals or alloys ; 

 consequently, we cannot obtain a balance by employing 

 two disks of different metals, and the instrument is so 

 sensitive to any variation in mass or matter that it 

 instantly detects the difference by clear loud tones on the 

 telephone. If I place two gold sovereigns of equal weight 

 and value, one in each coil, there is complete silence, 

 indicating identity or equality between them. But if one 

 of them is a false sovereign, or even gold of a different 

 alloy, the fact is instantly detected by the electrical 

 balance being disturbed. The instrument thus becomes 

 a rapid and perfect coin detector, and can test any 

 alloy, giving instantly its electrical value. The exceeding 

 sensitiveness of this electrical test I shall demonstrate by 



experiment this evening. Again, as regards coins, it 

 resolves an almost magical problem. Thus, if a person 

 puts one or several coins into one pair of coils, the amount 

 or nominal value being unknown to myself, I have only 

 to introduce into the opposite coils, different coins suc- 

 cessively, as I should weights in a scale, and when perfect 

 balance is announced by the silence, the amount in one 

 box will not only be the same nominal value, but of the 

 same kind of coin. 



4. We find by direct experiment with this instrument 

 that the preceding results are due to electric currents, 

 induced by the primary coil, and that it is by the reaction 

 of these that the balance is destroyed, for, if we take an 

 insulated spiral disk or hehx of copper wire, with its 

 terminal wires open, there is no disturbance of the 

 balance whatever, notwithstanding that we have intro- 

 duced a comparatively large amount of copper wire ; but 

 on closing the circuit the balance is at once very power- 

 fully disturbed. 



If the spiral is a flat one, resembling a disk of metal, 

 and circuit closed, we find that loud tones result when 

 the spiral is placed flat, or when its wire is parallel to 

 those on coils ; but if it is held at right angles to these 

 wires no sound whatever is heard, and the balance re- 

 mains perfect. The same thing occurs with disks of all 

 non-magnetic metals, and a disk of metal placed perpen- 

 dicular to the coils exerts no influence whatever. The 

 contrary result takes place with a spiral of iron wire or 

 disk of iron, the induced current circulating in the spiral 

 is at its maximum when the spiral lies flat or parallel 

 with the coils, giving no induced current whatever when 

 at right angles, but the disturbances of the induction- 

 balance is more than four-fold when perpendicular to the 

 wires of the coils, than when parallel with the same. 

 That this result is simply due to the property of magnetic 

 bodies of conduction of magnetism, we shall see in some 

 following experiments. 



That the cunents in non-magnetic metals travel in a 

 circle corresponding to that of the primary coil, may be 

 seen with spongy gold. In its first extremely divided 

 state it falls below our zero of hearing, on slightly shak- 

 ing the bottle we have 2° as its value ; on pressing it its 

 value rapidly increases with the pressure, until when 

 formed into a solid disk its value becomes 117°. 



5. The instrument proves that a very remarkable dif- 

 ference exists in bars of iron of the same exact form and 

 size, but of different provenance or treated in a different 

 manner ; in point of fact, no two bars, cut off of the same 

 rod, and treated alike, are exactly of the same value, or 

 induce a complete balance. 



Mr. Stroh, the eminent instrument-maker, has kindly 

 furnished me with numerous samples, varying in value in 

 degrees of the sonometer from 100 to 160. 



Chemically pure iron was found to be the best, but 

 still very shghtly superior to ordinary iron, which had 

 been drawn into a wire of the required thickness. The 

 fibrous condition thus developed being highly favourable 

 (if softened by heat) for the conduction of magnetism. 

 From numerous examples I select a few indicative 

 values : — 



Softened. Tempered. 



Chemically pure iron 160 130 



Forged soft iron 15° '25 



Wire-drawn iron 156 '20 



Cast steel 120 100 



6. As yet the instrument has given no indications of 

 molecular change produced by magnetism in non-magnetic 

 bodies, but the great change which takes place in all 

 magnetic bodies, except hard-tempered cast steel, indi- 

 cates that a molecular change of structure, analogous to 

 that of tempering, takes place upon iron, steel, and nickel. 



If we place a disk of iron in one of the coils, wc find that 

 the balance is destroyed, and that the iron has weakened 

 the induction by the absorption of work done in inducing 



