1911 Encyclopædia Britannica/Ohmmeter
OHMMETER, an electrical instrument employed for measuring insulation-resistance or other high electrical resistances. For the purpose of measuring resistances up to a few thousand ohms, the most convenient appliance is a Wheatstone’s Bridge (q.v.), but when the resistance of the conductor to be measured is several hundred thousand ohms, or if it is the resistance of a so-called insulator, such as the insulating covering of the copper wires employed for distributing electric current in houses and buildings for electric lighting, then the ohmmeter is more convenient. An ohmmeter in one form consists of two pairs of coils, one pair called the series coil and the other called the shunt coil. These coils are placed with their axes at right angles to one another, and at the point where the axes intersect a small pivoted needle of soft iron is placed, carrying a longer index needle moving over a scale.
Suppose it is desired to measure the insulation-resistance of a
Fig. 1.
system of electric house wiring; the ohmmeter circuits are then joined
up as shown in fig. 1, where W represents a portion of the wiring
of the building and I a portion of the insulating materials surrounding
it. The object of the test is to discover the resistance of the insulator
I, that is, to determine how much current flows through this insulator
by leakage under a certain electromotive force or voltage which must
not be less than that which will be employed in practice when the
electric lights supplied through these wires are in operation.
For this purpose the ohmmeter is provided with a small dynamo D,
contained in a box, which produces a continuous electromotive
force of from 200 to 500 volts
when the handle
of the instrument is
steadily turned. In
making the test, the
whole of the copper
wires belonging to any
section of the wiring and
the test must be connected
together at some
point and then connected
through the series
coil of the ohmmeter
with one terminal of the
dynamo. The shunt coil
Sh and the series coil Se
are connected together
at one point, and the
remaining terminals of
the dynamo and shunt coil must be connected to a “good
earth,” which is generally the gas or water pipes w of the
building. On setting the dynamo in operation, a current passes
through the shunt coil of the ohmmeter proportional to the voltage
of the dynamo, and, if there is any sensible leakage through the
insulator to earth, at the same time another current passes through
the series coil proportional to the conductivity of the insulation of
the wiring under the electromotive force used. The two coils, the
shunt and the series coil, then produce two magnetic fields, with
their lines of force at right angles to one another. The small pivoted
iron needle ns placed in their common field therefore takes up a
certain position, dependent on the relative value of these fields.
The tangent of the angle of deflection θ of this needle measured from
its position, when the shunt coil is disconnected, is equal to the ratio
of the voltage of the dynamo to the current through the insulator. If
we call this last resistance R, the voltage of the working dynamo V,
and the current through the insulator C, then tan θ＝C/V＝R.
Hence the deflection of the needle is proportional to the insulation
resistance, and the scale can be graduated to show directly this
resistance in megohms.
The Evershed and Vignoles form of the instrument is much used in testing the insulation resistance of electric wiring in houses. In this case the dynamo and ohmmeter are combined in one instrument. The field magnet of the dynamo has two gaps in it. In one the exciting armature is rotated, producing the working voltage of 250, 500 or 1000 volts. In the other gap are pivoted two coils wound on an iron core and connected at nearly a right angle to each other. One of these coils is in series with the armature circuit and with the insulation or high resistance to be measured. The other is a shunt across the terminals of the armature. When the armature is rotated, these two coils endeavour to place themselves in certain directions in the field so as to be perforated by the greatest magnetic flux. The exact position of the core, and, therefore, of an index needle connected with it, is dependent on the ratio of the voltage applied to the terminals of the high resistance or insulator and the current passing through it. This, however, is a measure of the insulation-resistance. Hence the instrument can be graduated to show this directly.
In the Nalder ohmmeter the electrostatic principle is employed. The instrument consists of a high-voltage continuous-current dynamo which creates a potential difference between the needle and the two quadrants of a quadrant electrometer (see Electrometer). These two quadrants are interconnected by the high resistance to be measured, and, therefore, themselves differ in potential. The exact position taken up by the needle is therefore determined by the potential difference (P.D.) of the quadrants and the P.D. of the needle and each quadrant, and, therefore, by the ratios of the P.D. of the ends of the insulator and the current flowing through it, that is, by its insulation resistance.
The ohmmeter recommends itself by its portability, but in default of the possession of an ohmmeter the insulation-resistance can be measured by means of an ordinary mirror galvanometer (see Galvanometer) and insulated battery of suitable voltage. In this case one terminal of the battery is connected to the earth, and the other terminal is connected through the galvanometer with the copper wire, the insulation of which it is desired to test. If any sensible current flows through this insulator the galvanometer will show a deflection.
The meaning of this deflection can be interpreted as follows: If a galvanometer has a resistance R and is shunted by a shunt of resistance S, and the shunted galvanometer is placed in series with a large resistance R′ of the order of a megohm, and if the same battery is applied to the shunted galvanometer, then the current C passing through the galvanometer will be given by the expression
C＝SVR′(R+S)+RS′
where V is the electromotive force of the battery. It is possible so to arrange the value of the shunt and of the high resistance R′ that potentiometer (q.v.). 
Fig. 2.the same or nearly the same deflection of the galvanometer is obtained as when it is used in series with the battery and the insulation-resistance. In these circumstances the current passing through the galvanometer is known, provided that the voltage of the battery is determined by means of a
Hence the resistance of the insulator can be ascertained, since it is expressed in ohms by the ratio of the voltage of the battery in volts to the current through the galvanometer in amperes. In applying this method to test the insulation of indiarubber-covered or of insulated copper wire, before employing it for electrical purposes, it is usual to place the coil of wire W (fig. 2) in an insulated tank of water T, which is connected to one terminal of
the insulated battery B, the other terminal being connected to the metallic conductor CC of the wire under test, through a galvanometer G. To prevent leakage over the surface of the insulating covering of the wire which projects above the surface of the water, it is necessary to employ a “guard wire” P, which consists of a piece of fine copper wire, twisted round the extremity of the insulated wire and connected to the battery. This guard wire prevents any current which leaks over the surface of the insulator from passing through the galvanometer G, and the galvanometer indication is therefore only determined by the amount of current which passes through the insulator, or by its insulation-resistance.
For further information on the measurement of high resistance, see J. A. Fleming, A Handbook for the Electrical Laboratory and Testing Room (2 vols., London, 1904); H. R. Kempe, A Handbook of Electrical Testing (London, 1900); H. L. Webb, A Practical Guide to the Testing of Insulated Wires and Cables (New York, 1902). (J. A. F.)