Fig. 16a.— Receiver in Syntonic Jar Experiment,
with Knob Coherer and Tapper-back ([cf. Fig. 4]).
The experiment of the syntonic Leyden jars can be conveniently shown with the double-knob or 1889 coherer. The pair of knobs are arranged to connect the coatings of the receiving jar (a large condenser being interposed to prevent their completing a purely metallic circuit), and in circuit with them is a battery and a bell. Every time the receiving jar responds syntonically to the electric vibration of the other jar, the knobs cohere (if properly adjusted) and the bell rings. If the bell is free in air it continues ringing until the knobs are gently tapped asunder; but if the bell stands on the same table as the knobs, especially if it rests one foot on the actual stand, then its first stroke taps them back instantly and automatically, and so every discharge of the sending jar is signalled by a single stroke of the bell. Here we have in essence a system of very distinctly syntonic telegraphy, for the jars and their circuits must be accurately tuned together, if there is to be any response. A very little error in tuning, easily made by altering the position of the slider ([Fig. 4]), will make them quite unresponsive, unless the distance between them is reduced.
Fig. 17.— Early Form of Coherer, consisting of a spiral of thin iron wire mounted on an adjustable spindle and an aluminium plate. When the lever is moved clockwise the tip of the iron wire presses gently against the aluminium plate, whose end is bent at right angles and passed through into the hollow circular wooden box, of which the upper figure shows the top and general appearance, and the lower figure shows the inside.
At the maximum distance of response the tuning required is excessively sharp. But, certainly, for these closed and durably-vibrating circuits, the distance of response is small, as has been said before. [Fig. 16a] shows the syntonic Leyden jar experiment arranged with the double knob coherer, instead of with the spark gap of [Fig. 4].
Coherer in open, responding to Feeble Stimuli:—
Small Sphere, Gas-lighter, Distant Sphere, Electrophorus.
Well, this arrangement, which I call a coherer, is the most astonishingly sensitive detector of Hertz waves. It differs from an actual air gap in that the insulating film is not really insulating; the film breaks down not only much more easily, but also in a less discontinuous and more permanent manner, than an air gap. Branly’s tube of filings, a series of bad contacts, clearly works on the same plan; and though a tube of filings is by no means so sensitive, yet it is in many respects easier to work with, and except for very feeble stimuli, is more metrical. If the filings used are coarse, say turnings or borings, the tube approximates to a single coherer; if they are fine, it has a larger range of sensibility. In every case what these receivers feel are sudden jerks of current; smooth sinuous vibrations are ineffective. They seem to me to respond best to waves a few inches long, but doubtless that is determined chiefly by the dimensions of some conductor with which they happen to be associated. ([Figs. 17] and [18].)
Fig. 18.— Early Form of Iron Borings Tube. One-half natural size, with solid brass cylinder terminals in each end of the tube, making contact with the borings.