By F.R. UPTON.
In this paper, read before the British Association, the author explained that the "Telemeter System," invented by C.L. Clarke, of New York, is a method by which the slow movement of a revolving hand of any indicating instrument may be reproduced by the movement of a similar hand at a distant place, using electricity to convey the impulse. The primary hand moves until it makes electrical contact, thus sending an impulse. It is here that all previous methods have failed. This contact should be absolute and positive, for if it is not, the receiver will not work in unison. The contact could often be doubled by the jarring of the instrument, thus making the receiver jump twice. Clarke has overcome this defect by so arranging his mechanism that the faintest contact in the primary instrument closes two platinum points in multiple arc with it, thus making a firm and positive contact, which is not disturbed by any jar on the primary contact. This gives the instruments a positive start for the series of operations, instead of the faint contact which would be given, for example, by the light and slowly moving hand of a metallic thermometer.
The other trouble with previous methods was that the contact points would corrode, and, in consequence of such corrosion, the instrument would fail to send impulses. Corrosion of the contacts is due to breaking the circuit slowly on a small surface. This is entirely remedied by breaking the circuit elsewhere than at the primary contact, using a quick motion, and also by giving this breaking contact large surface and making it firm. The instrument, as applied to a thermometer, is made as follows: From the free end of the light spiral of a metallic thermometer fixed at the other end, an arm, C, is attached, the end of which moves over an arc of a circle when the temperature varies. This end carries on either side of its extremity platinum contacts which, when the thermometer is at rest, lie between two other platinum points, A B, carried on radial arms. Any variation in temperature brings a point on the thermometer arm in contact with one of these points, and thus gives the initial start to the series of operations without opposing any friction to the free motion of the instrument. The first result is the closing of a short circuit round the initial point of contact, so that no current flows through it. Then the magnets which operate one set of pawls come into play.
The two contact points are attached to a toothed wheel in which the pawls play, and these pawls are so arranged that they drive the wheel whenever moved by their magnets; thus the primary contact is broken.
In the receiver there is a similar toothed wheel carrying the hand of the indicating instrument, and actuated at the same moment as the transmitter. The primary contacts are so arranged that the contact is made for each degree of temperature to be indicated. This series of operations leaves the instruments closed and the pawls home in the toothed wheel. To break the circuit another wire and separate set of contacts are employed.
These are arranged on the arms carrying the pawls, and so adjusted that no contact is made until after the toothed wheel has moved a degree, when a circuit is closed and a magnet attracts an armature attached to a pendulum. This pendulum, after starting, breaks the circuit of the magnets which hold the pawls down, as well as of the short-circuiting device. As the pendulum takes an appreciable time to vibrate, this allows all the magnets to drop back, and breaks all circuits, leaving the primary contacts in the same relation as at first. The many details of the instruments are carefully worked out. All the contacts are of a rubbing nature, thus avoiding danger from dirt, and they are made with springs, so as not to be affected by jar.
The receiving instruments can be made recorders also by simple devices. Thus, having only a most delicate pressure in the primary instrument, a distinct ink record may be made in the receiver, even though the paper be rough and soft. The method is applicable to steam gauges, water indicators, clocks, barometers, etc., in fact, to any measuring instrument where a moving hand can be employed.