The Standard Electrical Dictionary Part 90

Keeper.

A bar of soft iron used to connect the opposite poles of a horseshoe magnet or the opposite poles of two bar magnets placed side by side. It is designed to prevent loss of magnetism. The armature of a horseshoe magnet is generally used as its keeper. For bar magnets a keeper is used for each end, the magnets being laid side by side, with their poles in opposite direction but not touching, and a keeper laid across at each end connecting the opposite poles.

Kerr Effect.

The effect of an electrostatic field upon polarized light traversing a dielectric contained within the field. (See Electrostatic Refraction.)

Kerr's Experiment.

Polarized light reflected from the polished face of a magnet pole has its plane of polarization rotated; when it is reflected from the north pole the rotation is from left to right.

313 STANDARD ELECTRICAL DICTIONARY.

Key.

A switch adapted for making and breaking contact easily when worked by hand, as a Morse telegraph key.

Key Board.

A board or tablet on which keys or switches are mounted.

Key-board.

(a) A switch board, q. v.

(b) A set of lettered keys similar to those of a typewriter employed in some telegraph instruments. As each key is depressed it produces the contact or break requisite for the sending of the signal corresponding to the letter marked upon the key. The signal in printing telegraphs, on which such key-boards are used, is the reprinting of the letter at the distant end of the line.

Key, Bridge.

A key for use with a Wheatstone Bridge, q.v. It is desirable to first send a current through the four arms of the bridge in using it for testing resistances and then through the galvanometer, because it takes a definite time for the current to reach its full strength. This is especially the case if the element being measured has high static capacity, as a long ocean cable. If the galvanometer connections were completed simultaneously with the bridge connections a momentary swing would be produced even if the arms bore the proper relation to each other. This would cause delay in the testing. A bridge key avoids this by first connecting the battery circuit through the arms of the bridge, and then as it is still further depressed the galvanometer circuit is completed.

314 STANDARD ELECTRICAL DICTIONARY.

Fig. 206. CHARGE AND DISCHARGE KEY

Key, Charge and Discharge.

A key for use in observing the discharge of a condenser immediately after removing the battery. In one typical form it has two contacts, one below and one above, and being a spring in itself is pressed up against the upper one. Connections are so made that when in its upper position it brings the two coatings of the condenser in circuit with the galvanometer. When depressed it does the same for a battery. In use it is depressed and suddenly released when the galvanometer receives the full charge, before there has been time for leakage. This is one method of connection ill.u.s.trating its principle.

In the cut L is the spring-key proper. S2, is the upper contact screw against which the spring normally presses. In this position the galvanometer G is in circuit with the opposite coatings of the condenser C. On depressing the contact S2, is broken and S1, is made. This brings the battery B in circuit with the condenser coatings. On releasing the key it springs up and the galvanometer receives the effect of the charge of the condenser as derived from the battery.

Key, Double Contact.

A key arranged to close two distinct circuits, holding the first closed until the second is completed. It is used for Wheatstone bridge work.

Key, Double Tapper.

A telegraph key giving contacts alternately for currents in opposite directions, used in needle telegraphy.

Key, Increment.

A key for use in duplex and quadruplex telegraphy. Its action is to increase the line current, not merely to suddenly turn current into it.

315 STANDARD ELECTRICAL DICTIONARY.

Fig. 207. KEMPE'S DISCHARGE KEY.

Key, Kempe's Discharge.

A key giving a charging, discharging and insulating connection, for static condenser work. Referring to the cut l is a lever or spring with upper discharging contact s, and lower charging contact s'. In use it is pressed down by the insulating handle or finger piece C, until caught by the hook attached to the key I. This hook is lower down than that on the key D, and holds it in contact with the charging contact piece S'. On pressing the key I, marked or designated "Insulate," it springs up, breaks contact at S', and catching against the hook on D, which key is designated "Discharge," remains insulated from both contacts; next on pressing D it is released and springs up and closes the discharge contact S. It is a form of charge and discharge key. (See Key, Charge and Discharge.)

Key, Magneto-electric.

A telegraph key whose movements operate what is virtually a small magneto-generator, so as to produce currents of alternating direction, one impulse for each motion of the key. It is employed for telegraphing without a line battery, a polarized relay being used. In one very simple form a key is mounted on a base with a permanent magnet and connected to the armature, so that when the key is pressed downwards it draws the armature away from the poles of the magnet. If the magnet or its armature is wound with insulated wire this action of the key will cause instantaneous currents to go through a circuit connected to the magnet or armature coils.

Fig. 208. SIEMENS' MAGNETO-ELECTRIC KEY.

In Siemens & Halske's key an H armature E is pivoted between the poles N S, of a powerful compound horseshoe magnet, G G. It is wound with fine wire and a key handle H is provided for working it. In its normal position the handle is drawn upward, and the end S S of the armature core is in contact with the south pole S of the permanent magnet, and the end D D with the north pole. This establishes the polarity of the armature. On depressing the key the contacts are broken and in their place the end D D comes in contact with the south pole and the end S S with the north pole. This suddenly reverses the polarity of the armature and sends a momentary current through the armature coil which is in circuit with the line. The cut only shows the principle of the key, whose construction is quite complicated.

316 STANDARD ELECTRICAL DICTIONARY.

Key, Make and Break.

An ordinary electric key, usually making a contact when depressed, and rising by spring action when released, and in its rise breaking the contact.

Fig. 209. PLUG KEY

Key, Plug.

An appliance for closing a circuit. Two bra.s.s blocks are connected to the terminals, but are disconnected from each other. A bra.s.s plug slightly coned or with its end split so as to give it spring action is thrust between the blocks to complete the circuit. It is used in Resistance coils and elsewhere. (See Coil, Resistance.) Grooves are formed in the blocks to receive the plug.

Key, Reversing.

(a) A double key, arranged so that by depressing one key a current flows in one direction, and by depressing the other a current flows in the opposite direction. It is used in connection with a galvanometer in experimental, testing or measuring operations.

(b) A key effecting the same result used in quadruplex telegraphy.

Key, Sliding-Contact.

A name given to the key used for making instantaneous contacts with the metre wire of a metre bridge, q. v. The name is not strictly correct, because it is important that there should be no sliding contact made, as it would wear out the wire and make it of uneven resistance.