SCIENTIFIC SUMMARY. 
329 
These two pairs 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. 
The telephone is well adapted as an indicator, but not as a measurer of 
the forces brought into action. For this reason he has joined to this instru- 
ment an instrument to which he has 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 
communication 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 with- 
draw 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 absolute 
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 absolute zero of 
sound ; each degree that it is moved is accompanied by its relative degree of 
increase ; and this measure may be expressed in the degrees of the millimetres 
passed through, or by the square of the distances in accordance with the 
curve of electro-magnetic action. 
If we place in the coils of the induction balance a piece of metal, say 
copper, bismuth, or iron, we at once produce a disturbance of the balance, 
and it will give out sounds more or less intense on the telephone according 
to the mass, or if of similar sizes, according to the molecular structure of 
the metal. The volume and intensity of sound is invariably the same for a 
similar metal. If by means of the switching key the telephone is instantly 
transferred to the sonometer, and if its coil be at zero, we should hear sounds 
when the key is up or in connection with the induction balance, and no 
sounds or silence when the key is down or in connection with the sonometer. 
If the sonometer coil were moved through several degrees, or through more 
than the required amount, we should find that the sounds increase when the 
key is depressed ; but when the coil is moved to a degree where there is 
absolute equality with the key up or down, then the degree on scale should 
give the true value of the disturbance produced in the induction balance ; 
and this is so exact that if we put, say, a silver coin whose value is 115°, no 
other degree will produce equality. Once knowing, therefore, the value of 
any metal or alloy, it is not necessary to know in advance what the metal 
is, for if its equality is 115°, it is silver coin ; if 52, iron ; if 40, lead ; if 10, 
bismuth ; and as there is a very wide limit between each metal, the reading 
of the value of each is very rapid, a few seconds sufficing to give the exact 
sound value of any metal or alloy. 
