ELECTRO-CHEMICAL EQUIVALENT OF SILVER. 
4-19 
proper precautions this method of testing seems satisfactory, being approximately 
independent of the equality of mean radii of the coils compared. 
A second test was suggested and executed by Mr. Glazebrook, The two coils 
retaining a fixed position, the ratios of the self-inductions of each to the mutual in¬ 
duction of the pair were determined by Maxwell’s method.* These ratios, which 
should have been nearly equal, were found to differ considerably in the direction which 
showed a deficiency in the self-induction of the ebonite coil. 
After this it was no longer doubtful that the coil was defective. In unwinding it 
more than one bad place was detected, although the original winding had been carefully 
done under our own eyes. The ring was rewound with fresh wire on Nov. 30, 1882 ; 
and we were so much impressed with the necessity of a thorough check upon the 
insulation that we devised a delicate test similar, as we afterwards found, to one that 
had already been successfully used by Graham Bell.1 Four similar coils of fine wire, 
wound upon wood, and of the same mean diameter as the ebonite coil, were arranged 
so as to form a Hughes induction balance. The lower coils form a primary circuit, 
and are connected with a microphone clock or other source of variable current. The 
upper coils and associated telephone form a secondary current. The distance between 
the upper and lower coils is such as to allow the insertion of the ebonite coil between 
them, suitable support being provided for it to guard against displacement of the 
principal coils. If the distances of the four coils are adjusted by screw-motions 
to an exact balance, so that no sound is audible in the telephone (held at some 
distance away), the introduction of a tertiary circuit between one primary and 
secondary causes a revival of sound whose intensity depends upon the conductivity, 
&c., of the tertiary circuit. If the tertiary circuit consists of a single turn of wire, such 
as that on the ebonite ring, the sound heard is quite loud, and remains audible when 
a resistance of about 1 ohm is included. A single circlet of copper wire ‘004 inch 
diameter gives a very distinct sound. When the ebonite coil, with ends unconnected, 
is introduced, the sound is audible, but much less than that from the fine copper circlet. 
Part of this effect may be attributed to its finite capacity as a condenser, in virtue of 
which sound might be heard in any case ; but it is probable that the insulation is in 
reality somewhat imperfect. The closing of the circuit through a megohm gives a 
distinct augmentation of sound; and thus it is evident that the insulation, if not 
perfect, is at any rate abundantly sufficient for the purposes of the present 
investigation. 
The current weighing apparatus was set up in February, 1883, and worked satis¬ 
factorily from the first. Apart from errors in the constant of the instrument, the 
* ‘ Electricity,’ § 756. 
t “Upon tie Electrical Experiments to Determine the Location of the Bullet in the Body of the late 
President Garfield,” &c. A paper read before the American Association for the Advancement of Science, 
August, 1882. 
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