pivoted 1-500ths ( slide wire the base, a The mod ༢ 99 making contact with A B and B C at the points 1 b and c d, through knife edges if possible, with a view to clear definition. G is a high resistance galvanometer, ontrolled by means of the key K1, The principle of the est consists in selecting two points c and d on the tandard slide wire B C, such that a balance is obtained n the galvanometer when K and Kl are closed, then The Post Office form of Wheatstone bridge may be dapted to Thomson's method of low resistance measureent, as indicated by Fig. 46, the lettering being the me as in Fig. 45. The INF plug is removed for this est, and a subsidiary resistance f of known value is inroduced as shown, together with the extraneous key K. he remainder of the figure needs no explanation. Müller's and Wallau's Method of Comparing very ow Resistances is indicated in Fig. 47, where A and B re the two low resistances to be compared, connected in eries, and with the source of constant current E (a batry of accumulators answers the purpose). The termini f the resistances A and B as regards their precise value represented by the points a, b, c, and d. G is a alvanometer, one side of which is permanently conected, as shown, to the junction of the standard resistnces r and r1, which are made up in resistance box re E 2 the passage of an electric current. In Carey Foster's method these extra gaps are utilised as represented in the figure. The resistance to be measured, r, in this case bridges gap 3, whilst 1 and 2 are occupied by resistances whose ratio to one another does not differ from unity more than does the ratio of the resistance x to the total resistance of the slide wire A B. The testing battery E and the galvanometer G are connected as shown, gap 4 being bridged by its link of negligible resistance. The slider S is adjusted until a balance is obtained on the galvanometer G. The scale reading d is then noted, and x is disconnected from gap 3 and connected instead across gap 4, 3 being in this case bridged by its conducting link. A fresh balance is then obtained, and the second scale reading dl is noted, then x = dl d. It is, of course, essential to this test that the value of the scale reading in ohms or fractions of an ohm resistance of that section of the slide wire bounded by such scale reading be known. If such knowledge be not immediately available it may readily be attained by inserting a resistance of known value, such as lw, in the place of x, and similarly obtaining readings d and dl as before, then since x = dl — d, ·l∞ = dl d, and consequently the difference between dl and d will give the number of scale divisions corresponding to le, or that difference multiplied by ten will give the number n corresponding to one ohm, so that our formula, to read directly in ohms, becomes x = It is essential for the accurate conduct of this test that the conducting link and its concomitant connections should be massive, and of absolutely negligible resistance. Thomson's bridge method of determining low resistances is a modification of the first described fall of potential method, and is represented diagrammatically in Fig. 45, where A B is a circuit, the resistance of a section x of which, lying between the points a and b is required, whilst B C is a standard slide wire of known resistance per unit of length, joined in series with it, and a battery E and key K. The contact resistance at B is not of any import in this test. ef, el fl, are subsidiary resistances making contact with A B and B C at the points a b and c d, through knife edges if possible, with a view to clear definition. G is a high resistance galvanometer, controlled by means of the key Kl. The principle of the test consists in selecting two points c and d on the standard slide wire B C, such that a balance is obtained on the galvanometer when K and K1 are closed, then The Post Office form of Wheatstone bridge may be adapted to Thomson's method of low resistance measurement, as indicated by Fig. 46, the lettering being the same as in Fig. 45. The INF plug is removed for this test, and a subsidiary resistance f of known value is introduced as shown, together with the extraneous key K. The remainder of the figure needs no explanation. Müller's and Wallau's Method of Comparing very Low Resistances is indicated in Fig. 47, where A and B are the two low resistances to be compared, connected in series, and with the source of constant current E (a battery of accumulators answers the purpose). The termini of the resistances A and B as regards their precise value are represented by the points a, b, c, and d. G is a galvanometer, one side of which is permanently connected, as shown, to the junction of the standard resistances r and rl, which are made up in resistance box E 2 |