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OHMMETER

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Originally appearing in Volume V20, Page 35 of the 1911 Encyclopedia Britannica.
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OHMMETER ,an See also:

electrical See also: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 See also:Wheatstone's See also:Bridge (q.v), but when the resistance of the conductor to be measured is several See also:hundred thousand ohms, or if it is the resistance of a so-called insulator, such as the insulating covering of the See also:copper wires employed for distributing electric current in houses and 'buildings for electric See also:lighting, then the ohmmeter is more convenient. An ohmmeter in one See also:form consists of two pairs of coils, one pair called the See also: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 See also:needle of soft See also:iron is placed, carrying a longer See also:index needle moving over a See also:scale. Suppose it is desired to measure the insulation-resistance of a See also:system of electric See also:house wiring; the ohmmeter circuits are then joined up as shown in fig. 1, where W represents a portion of the wiring of the See also:building and I a portion of the insulating materials surrounding it. The See also:object of the test is to discover the resistance of the insulator I, that is, to determine how much current flows through this insulatorby leakage under a certain electromotive force or voltage which must not be less than that which will be employed in practice when the electric See also:lights supplied through these wires are in operation. For this purpose the ohmmeter is provided with a small See also:dynamo D, contained in a See also:box, uhich 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 See also: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 " See also:good See also:earth," which is generally the See also:gas or See also: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 See also: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 See also:fields, with their lines of force at right angles to one another.

The small pivoted iron needle ns placed in their See also:

common See also:field therefore takes up a certain position, dependent on the relative value of these fields. The tangent of the See also:angle of deflection 0 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 See also:call this last resistance R, the voltage of the working dynamo V, and the current through the insulator C, then tan B = 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 See also: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 See also:armature is rotated, producing the working voltage of 25o, 500 or woo volts. In the other See also:gap are pivoted two coils See also: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 See also: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 See also:place themselves in certain directions in the field so as to be perforated by the greatest magnetic See also: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 See also:

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 See also:default of the See also:possession of an ohmmeter the insulation-resistance can be measured by means of an See also:ordinary See also:mirror See also:galvanometer (see GALVANOMETER) and insulated See also: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 See also:wire, the insulation of which it is desired to test. If any sensible current flows through this insulator the galvano- See also:meter 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 See also:order of a megohm, and if the same W W See also:Pau, See also:Perigord and other cities were embodied in See also:forty-five MS. `volumes, which were sent by his son See also:Gabriel to See also:Colbert. Twenty-three of these are in the Bibliotheque Nationale of See also:Paris (See also:Coll.

See also:

OHNET" battery is applied to the shunted galvanometer, then the current C passing through the galvanometer will be given by the expression SV C R'(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 the same or nearly the same deflection of the galvanometer is obtained as when it is used in series with the battery and the insula- tion-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 See also:potentiometer (q.v.). 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 apply- See also:ing this method to lest the insulation of indiarubber - covered or of insulated copper wire, before employing it for electrical purposes, it is usual to place """1 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 he wire under' test, through a galvano- meter G. To prevent leakage over the See also: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 See also:fine copper wire, See also:twisted See also:round the extremity of the insu- lated wire and connected to the battery. This guard wire pre- vents 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 See also:information on the measurement of high resistance, see J. A. See also:Fleming, A Handbook for the Electrical Laboratory and Testing See also:Room (2 vols., See also:London, 1904) H. R.

See also:

Kempe, A Handbook of Electrical Testing (London, 1900) ; H. L. See also:Webb, A See also:Practical See also:Guide to the Testing of Insulated Wires and Cables (New See also:York, 1902). (J. A.

End of Article: OHMMETER

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