Fig. 252. THEORY OF OHMMETER.
Fig. 253. OHMMETER.
Ohmmeter.
An instrument for measuring directly the resistance of a conductor or of
any part of a circuit through which a strong current is passing. It is
the invention of Prof. W. E. Ayrton.
It contains two fixed coils at right angles to each other acting on the
same needle of soft iron. One coil is of thick wire and is placed in
series with the resistance to be measured. The other is of very thin
wire and is placed in parallel with the same resistance. One wire acts
by the total current, the other by the potential difference between the
ends of the resistance. The action on the soft iron needle is due to the
ratio of potential difference to total currents, or to the resistance
itself. By properly designing and proportioning the coils the angular
deflections of the needle are made proportional to the resistance.
In use the thick wire may be kept permanently in circuit. On connecting
the binding posts of the thin wire coil to any two parts of the circuit
its resistance is at once given by the deflection of the needle.
When no current is passing the needle rests in any position. A current
in the thick coil brings it to zero. A current simultaneously passing
through the thin high resistance coil brings about the deflection.
The instrument is a commercial rather than a scientific one.
Ohm's Law.
The fundamental law expressing the relations between current,
electro-motive force and resistance in an active electric circuit. It
may be expressed thus:
(a) The current strength is equal to the electro-motive force divided by
the resistance.
(b) The electro-motive force is equal to the current strength multiplied
by the resistance.
(c) The resistance is equal to the electro-motive force divided by the
current strength. All these are different forms of the same statement.
Algebraically the law is usually expressed thus, (a) C = E/R. It may
also be expressed thus: (b) E = C*R and (c) R= E/C, in which R denotes
resistance, C denotes current strength, and E denotes electro-motive
force.
Ohm, True.
The true ohm is the resistance of a column of mercury 1 square
millimeter in cross-sectional area, and 106.24 centimeters long. (See
Ohm.)
Synonym-Rayleigh Ohm.
Oil Insulation.
Oil insulation has received several applications in electrical work. It
has been proposed for use in underground conduits. These it was proposed
to fill with oil after the insertion of the conductors, the latter
properly wrapped with cotton or other covering. For induction coils it
has been very successfully used. Its principal utility depends on the
fact that it is liquid, so that if pierced by a spark it at once closes
again. A solid insulator if pierced is permanently injured. It is also
used in telegraph insulators (see Insulator, Liquid) to prevent surface
leakage.