Independently of the use which may be made of it for measuring time in absolute value, the apparatus described possesses peculiar properties.  It constitutes a kind of clock which indicates, registers, and, if needful, corrects automatically its own variations of speed.  The apparatus being regulated so that the magnetic needle may be at zero, if the speed of the commutator is slightly increased, the equilibrium is disturbed and the magnetic needle deviates in the corresponding direction; if on the contrary the speed diminishes, the action of the antagonistic circuit predominates, and the needle deviates in the contrary direction.  These deviations, when small, are proportional to the variations of speed.  They may be, in the first place, observed.  They may, further, be registered, either photographically or by employing a Redier apparatus, like that which M. Mascart has adapted to his quadrant electrometer; finally, we may arrange the Redier to react upon the speed so as to reduce its variations to zero.  If these variations are not completely annulled, they will still be registered and can be taken into account.

As an indicator of variations this apparatus can be of remarkable sensitiveness, which may be increased indefinitely by enlarging its dimensions.

With a battery of 10 volts, a condenser of a microfarad, 10 discharges per second, and a Thomson’s differential galvanometer sensitive to 10^{-10} amperes, we obtain already a sensitiveness of 1/1000000, i.e., a variation of 1/1000000 in the speed is shown after some seconds of a deviation of one millimeter.  Even the stroboscopic method does not admit of such sensitiveness.

We may therefore find, with a very close approximation, a speed always the same on condition that the solid parts of the apparatus (the condenser and the resistance) are protected from causes of variation and used always at the same temperature.  Doubtless, a well-constructed astronomical clock maintains a very uniform movement; but the electric clock is placed in better conditions for invariability, for all the parts are massive and immovable; they are merely required to remain unchanged, and there is no question of the wear and tear of wheel-work, the oxidation of oils, or the variations of weight.  In other words, the system formed by a condenser and a resistance constitutes a standard of time easy of preservation.

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NEW METHOD OF MAINTAINING THE VIBRATION OF A PENDULUM.

A recent number of the Comptes Rendus contains a note by M.J.  Carpentier describing a method of maintaining the vibrations of a pendulum by means of electricity, which differs from previous devices of the same character in that the impulse given to the pendulum at each vibration is independent of the strength of the current employed, and that the pendulum itself is entirely free, save at the point of suspension.  The vibrations are maintained, not by direct impulsion, but by a slight horizontal displacement of the point of suspension in alternate directions.