Electrical Devices and How They Work
��VI. — Electrical metering instruments By Peter J. M. Clute, B. E.
��ELECTRICITY is manifested to us only through certain effects which it produces. These effects may be mostly classified under chemical, thermal, magnetic, and static effects.
Chemical effect is manifested by the decomposition of a solution when a cur- rent of electricity passes through it.
Thermal effect is produced by the pas-
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���A gas voltameter consisting of two platinum electrodes immersed in acidulated water
sage of an electric current through a con- ductor of appreciable resistance.
Magnetic effect results from an electric current in a conductor when brought into a magnetic field, the field being produced by a magnetic or other electric currents.
Static effect is the attraction or re- pulsion existing between highly electrified bodies.
These effects all occur in perfect ac- cordance with definitely fixed natural laws, and as a consequence they have been utilized in the design of various instru- ments for the measurement of electrical qualities.
Electrolytic meters, depending on the chemical effect of the electric current, do not indicate directly in amperes the cur- rent flowing, but are used to determine the quantity of current which passes in a definite period of time. Thus, they should
��be termed coulomb, or ampere-hour, meters. In Fig. 1 is shown the gas volta- meter or electrolytic meter. The gas voltameter consists of two platinum electrodes immersed in acidulated water, so arranged that all the evolved gas would be collected in a graduated cylinder. It can thus be demonstrated that the amount of gas is entirely independent of any consideration by the quantity of cur- rent flowing. By using solutions of different metals for electrolj'te, it can be shown that the weight of metal deposited on the positive electrode is always pro- portional to the amount of current in the circuit.
The commercial electrolytic instrument depends on the principle that current passing through a volume of water de- composes it into its constituent gases, hydrogen and oxygen. This decomposi- tion is directly in proportion to the cur- rent passing, one ampere-hour decompos- ing .338 gms. of water. The current flows through a volume of water contained in a properly graduated tube, the change in volume of water in a given time, indi- cated by the difference in levels, showing the amount of current which has passed.
The commercial use of electrolytic meters is quite limited, and in most cases they have been superseded by direct- reading electro-magnetic type meters.
The thermal effect of electric current is utilized in some instruments to- measure the current passing, by means of the ex- pansion of a wire. The heating of a wire is proportional to the square of the cur- rent and the resistance of the wire, when a current is passed through it.
Hot-wire instruments are used to meas- ure current or differences in potential by the heating effect of the current. Refer- ring to Fig. 2, a long wire, of high re- sistance, non-oxidizable metal, has one end fastened to a plate, passes over a pulley and back to the plate. A spring attached to the plate keeps the wire stretched taut. When a current is passed through the wire, it heats and expands,
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