An elementary book on electricity and magnetism and their applications . rds the current flowing through both the unknown re-sistance and voltmeter. Compute the percentage error made in theammeter as thus connected. 3. In the slide-wire form of Wheatstone bridge shown in figure 119suppose # = ohms, AB = cm., and ra= cm. Computethe value of X. 192 ELECTRICITY AND MAGNETISM 4. In testing a Wheatstone bridge, 4-ohm and 5-ohm coils are in-serted in the loop AC and CB. Find the position which D should haveon the meter wire ADB. 5. If the resistance coils of a box form of bridge range
An elementary book on electricity and magnetism and their applications . rds the current flowing through both the unknown re-sistance and voltmeter. Compute the percentage error made in theammeter as thus connected. 3. In the slide-wire form of Wheatstone bridge shown in figure 119suppose # = ohms, AB = cm., and ra= cm. Computethe value of X. 192 ELECTRICITY AND MAGNETISM 4. In testing a Wheatstone bridge, 4-ohm and 5-ohm coils are in-serted in the loop AC and CB. Find the position which D should haveon the meter wire ADB. 5. If the resistance coils of a box form of bridge range from 1 to10,000 ohms, and the ratio coils are 1, 10, 100, and 1000 ohms, whatratio would you use to measure 1,000,000 ohms? 6. If the same box used in problem 5 were used to measure anunknown resistance of ohms, what must be the ratio ? 136. Locating faults in cables. There are two commonmethods of locating a ground in a cable, such as an under-ground telephone wire, and these are called the Murray loopand the Varley loop. Since both methods are simply modifica-. Fig. 126. — Connections for the Murray loop. tions of the Wheatstone bridge, the same underlying prin-ciples apply to them. These tests are called the loop tests, because the faulty wireis always connected at its distant end with a good wire and thetwo in combination make a loop. The loop is divided into twoparts by the fault, which here is assumed to be a ground. In the Murray loop arrangement (Fig. 120) these two parts, % andL—x, are two arms of the Wheatstone bridge, and the other two arms. A and B, are made up in the testing apparatus. The bridge is balancedby moving C or varying the ratio of .t and B until there is no defied ionof the galvanometer. The regular equation of the bridge gives ELECTRIC MEASUREMENTS 193 A_L-xB x when L is the entire length of the loop which is known, and X is thedistance out to the fault. From this we find b —) L. The Varley loop differs from the Murray in having a portion o
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