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the leading wires a b, it follows from the laws governing. multiple circuits that if we connect a resistance S across a and b at a point between the source of current and the
galvanometer, part of the current passing will go through S and part through the winding of G. In this capacity S constitutes what is known as a shunt to the galvanometer G, and, by the proper apportioning of the value of S in terms of the ohm or unit of resistance, having due regard to the like fixed resistance of G, we can so control matters that we know exactly what fraction of the total current passing from a to b is going through G, and from this fact we can in consequence calculate the total current passing. It will readily be seen from this that by making S adjustable within the required limits, the current actually passing round the coils of the galvanometer G can be reduced to a practical working limit, thus increasing the direct range of the instrument.
The shunts usually employed are three in number, viz., 1-9th, 1-99th, and 1-999th part of the total resistance of the galvanometer which is in circuit at the time. These three shunts respectively reduce the currents flowing through the instruments when they are severally inserted, to the 1-10th, 1-100th, and 1-1,000th parts of the total current flowing in the circuit, and they are usually marked and known as the 1-10th, 1-100th, and 1-1,000th shuffle. They are wound non-inductively, and are usually made up in box form, as illustrated in the accompanying sketch, the respective extremities of the coils being brought up to suitable metal blocks on the insulating lid, and provided with openings for a plug, by the manipulation of which they can be placed in circuit or withdrawn as desired.
Without entering into theory, the writer would call the attention of readers to the fact that the formula for calculating the true galvanometer reading is as follows: Actual reading on scale multiplied by Resistance of Galvanometer + res. of Shunt.
Resistance of Shunt. The latter fraction, generally simplified to the initial
G + S letters tbus :
is technically known as the
S multiplying power of the shunt, and is a useful formula to bear in mind.
From the foregoing context it will be obvious that galvanometers having different resistances require special shunt boxes, wound to such values as are indicated by the fractions 1-9th, 1-99th, and 1-999th, previously alluded to, or, in other words, every galvanometer requires its own set of shunts. To overcome this obvious disadvantage, another system of shunts has been introduced of late years, and is being generally adopted with up-to-date instruments. It is known as the “Universal Shunt System,” in that the same set of shunts can be applied universally to each and every galvanometer within certain practical limits; its principle is represented diagrammatically in Fig. 11, where G represents the galvanometer as before, and S the shunt resistance, which is, however, in this case, permanently connected across the galvanometer terminals.
a and 6 are, as before, the wires leading in from the current source,
of which is directly connected to the
Fig. 11. galvanometer and one extremity of
of S, whilst b is by suitable means made adjustable with regard to its point of contact with S. By a proper appor. tioning of the contact positions on S, which detail, as it involves certain theoretical explanations, we will not enter upon here, the same result is obtained as in the ordinary shunt system, viz., the passage of 1-10th, 1-100th, or 1-1,000th part, as the case may be, of the total current, through the galvanometer G, whilst the attendant advantages of adaptability to instruments of varying resistance are too apparent to need further comment here.
The above system has been successfully applied to a special switch combination for insulation testing, which will be described later.
The Wheatstone Bridge.-We now come to a consideration of another very important item in an electrical testing installation, viz., the Wheatstone Bridge, or set of proportional resistances, which is a necessary adjunct to the great majority of electrical tests.
In order to describe the Wheatstone bridge proper, it is necessary in the first instance for the reader to gain an insight into the principles underlying its utilisation for testing purposes, and to this end we will refer to Fig. 12, which will be a familiar diagram to the majority of readers. A, B, C, D is a parallelogram, representing the ultimate disposition of four distinct resistances to the passage of an
electric current. Across the points B and D is connected a galvanometer G, and across A and C the testing battery E. We will represent the resistances A B, B C, C D, and D A by the letters a, b, c, and d respectively, then, when no current flows through the galvanometer G under the conditions depicted above, i.e., when B and D are at the same potential a:d::b:c, a simple proportion, into the theore tical explanation of which we will not enter here; suffice it to say that it is so, and that the combination forms a very useful arrangement for the measurement of resistances, a and d being commonly known as the “proportional arms,” b as the "adjustable arm ” of the bridge re spectively, and c being usually filled by the unknown resistance which it is required to measure.
The Wheatstone bridge, as commonly constructed for practical use, consists of a series of coils of platinoid wire wound non-inductively, i.e, the wire is doubled back upon itself and wound double upon the insulating bobbin ; by this means any inductive effects due to a current flowing through one half of the winding will be counteracted by the same current when flowing back along the contiguous path provided for it. The extremities of the coils which are accurately wound to the required resistance by comparison with certain standards, are brought up to massive brass blocks screwed to the upper surface of a slab of ebonite which forms the lid of the containing box. The blocks are shaped as in Fig. 13, which is a side view of the ebonite lid, and some of the coils as they appear when removed from
The object of the chamfering on the underside
of the blooks is to allow clearance for the removal of dust and dirt, which will collect even under the most favourable conditions on the surface of the ebonite, and tend, by its presence at this point, to provide a path of low resistance for the current, thereby leading to errors in the ultimate measurements taken by means of the bridge. The plugs, of which a sample is depicted above, consist of a tapered brass shank screwed and pinned into an ebonite crown of the shape exhibited in the figure, which serves as a handle for its manipulation. They fit into recesses between tw contiguous blocks bored to receive them, and are accurately ground into place to ensure a perfect fit, so that no extra resistance may be introduced by loose contacts between plug and block. The office performed by a plug when inserted between two neighbouring blocks is to short circuit the resistance coil connected to them and thereby cut it out of circuit by a path of negligible resistance. A diagrammatio plan of the complete bridge, with the values usually given to the various coils is represented in Fig. 14, whilst the complete article is depicted in general view by the subsequent illustration.
The letters a b and d in Fig. 14 also represent the arms corresponding with those similarly lettered in the original Fig. 12, whilst the small circles represent the terminals usually provided on the bridge.