The same instrument is wound for high resistance and const.i.tutes a Magnetic Vane Voltmeter.

28 STANDARD ELECTRICAL DICTIONARY.

Ammeter, Magnifying Spring.

A solenoid ammeter in which a spiral spring is used to convert the longitudinal motion of the armature or movable core into a rotary motion (see Ammeter, Ayrton's) and magnify the apparent range of motion.

Ammeter, Permanent Magnet.

An ammeter with a magnetic field produced by a permanent magnet.

Ammeter, Solenoid.

An ammeter in which the attraction, when a current is pa.s.sing through it, exerted by a hollow coil of wire upon an iron bar or tube in line with its axis, is utilized to indicate the strength of current. The bar is drawn into the coil to different extents proportional to the attraction. As an example see Ammeter, Ayrton's, and cut of Gravity Ammeter.

Ammeter, Spring.

An ammeter in which the part moved by the current is controlled or brought to the zero position by a spring.

Ammeter, Steel Yard.

A solenoid ammeter in which the solenoid core is suspended vertically from the short end of a steel yard fitted with a sliding weight. The current pa.s.ses through the solenoid coil and attracts or draws downwards the coil. A sliding weight is moved in and out on the long steel-yard arm which is graduated for amperes. In use the weight is slid out until the arm is in equipose; the divisions give the amperes.

Fig. 12. STEEL YARD AMMETER.

29 STANDARD ELECTRICAL DICTIONARY.

Ammunition Hoist, Electric.

An apparatus for use on ships for hoisting ammunition to the guns by an electric elevator. The characteristic feature of it is that a constant motion of the switch or handle is required to keep it in action. If the operator is shot so as to be incapacitated from taking charge of the switch, the hoist stops until another is a.s.signed to it.

Amperage.

Current intensity expressed in amperes, as an amperage of ten amperes.

Ampere.

The practical unit of electric current strength. It is the measure of the current produced by an electro-motive force of one volt through a resistance of one ohm. In electric quant.i.ty it is the rate of one coulomb per second. It is one-tenth the absolute C. G. S. unit of current strength. Its best a.n.a.logy is derived from water. a.s.suming the electric current to be represented by a current of water, the pressure, head, or descent producing such current would be the electro-motive force. The current might be measured in gallons (or other unit) pa.s.sed per second. In the a.n.a.logy these gallons would be coulombs. But it might be measured by reference to a standard stream, as for instance, the stream which would pa.s.s through a hole an inch square under a given head, say six inches of water. This unit is the miner's inch, and is the exact a.n.a.logy of the ampere. A current of water may flow at the rate of so many miner's inches, just as a current of electricity may flow at the rate of so many amperes. In neither case it will be noted is there any reference to time. "An ampere per second" is a redundant expression, and means no more than "an ampere"; an "ampere-second," on the other hand, is a coulomb. The number of coulombs pa.s.sed per second gives the amperes of current.

For value of ampere, see Coulomb.

[Transcriber's note: The SI definition of an ampere: A current in two straight parallel conductors of infinite length and negligible cross-section, 1 metre apart in vacuum, would produce a force equal to 2E-7 newton per metre of length.]

Fig. 13. THE MINER'S INCH AS AN a.n.a.lOGY FOR THE AMPERE.

30 STANDARD ELECTRICAL DICTIONARY.

Ampere, Arc.

A conductor bent into the arc of a circle, and employed in measuring the electric current by the electric balance.

Ampere-currents.

The currents a.s.sumed to be the cause of magnetism. (See Magnetism, Ampere's Theory of.)

Ampere-feet.

The product of amperes of current by the length, in feet, of a conductor pa.s.sing such current. It may be in empiric calculations of dynamo or motor construction, but is little used. One ampere-foot is a current of one ampere pa.s.sing through one foot length of a conductor, or one-tenth ampere through ten feet, and so on.

Ampere-hour.

The quant.i.ty of electricity pa.s.sed by a current of one ampere in one hour. It is used by electric power and lighting companies as the unit of energy supplied by them, because they maintain a constant potential difference in their leads, so that only the amperes and hours need measuring or recording to give the energy, viz. : volt-ampere-hours.

The same unit is applied to batteries to indicate their potential energy, because they also are a.s.sumed to be of constant voltage or electro-motive force.

Ampere-meters.

The product of amperes of current by the length, in meters, of a conductor carrying such current. One ampere-meter is a current of one ampere pa.s.sing through one meter of a conductor.

The term must not be confused with the identically spelled Ampere-meter, a synonym for Ammeter.

Ampere-minute.

The quant.i.ty of electricity pa.s.sed by a current of one ampere in one minute; sixty coulombs.

Ampere Ring.

A conductor forming a ring or circle used in electric balances for measuring currents. (See Balance, Ampere.)

Ampere-second.

The quant.i.ty of electricity pa.s.sed by a current of one ampere in one second; the coulomb, q. v.

Amperes, Lost.

In a shunt or compound-wound dynamo, part of the total amperes of current produced in the armature coils go through the shunt, and hence, do not appear in the outer circuit. S. P. Thompson has proposed the term "lost amperes" for this portion of the current.

Ampere's Memoria Technica.

An expression of the effect of a current on a magnetic needle. If we imagine the observer in the line of the current and facing the magnetic needle, the current entering by his feet and leaving by his head, the north pole is deflected to his left.

31 STANDARD ELECTRICAL DICTIONARY.