Patent GB-365138-A (GB)
Abstract: 365,138. Gauging-apparatus. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Waite, B. C. ; 1409, Regent Street, and Mershon, A. V.; 823, State Street, both in Schenectady, New York, U.S.A.) Sept. 12, 1930, No. 27322. Convention date, Sept. 12, 1929. [Class 106 (ii).] Relates to electrical a p - paratus of a t y p e utilizing two normally balanced d if - ferential alternating current circuits and two comparison electromagnets one in each circuit and means whereby upon the balance of the circuits being broken the consequential flow of current operates an indicating device, and consists in providing two movable magnetic cores, a coil assembled on each of said cores, means to adjust and lock in place said cores, a stationary armature acting as part of the magnetic circuit of one core, and movable gauging means for varying the magnetic reluctance of the other core. A motor generator 10 supplies single phase alternating current to the primary 12 of a step down transformer the secondary 13 of which is connected to the connecting leads of pairs of oppositely-wound coils 17, 18 and 14, 15 of which the latter are wound on the same core of a transformer 16. Each coil of a pair is similar to the other and they are connected as shown to form two differentially balanced circuits. An ammeter 20 is connected at the points 21, 22 through a rectifier 19 and reads zero. If the magnetic reluctance say of the coil 18 is altered, the balance is upset and the needle of the ammeter is deflected from zero. In the actual measuring instrument, Fig. 3, the coils 17, 18 are adjustably mounted in a nonmagnetic cover 41 of the metallic case 29. A stationary bar 28 of magnetic material forms part of the magnetic circuit of the coil 17 while the end 54 of a lever 30 mounted on a pivot 31 forms part of the magnetic circuit of the coil 18. The lever 30 is held by springs 35, 36, 37 in a state of equilibrium yet permitting positive and lively action in either direction. The short arm of the lever carries at the end a contact stud 32 having a diamond point 33. The projecting end of the casing is provided opposite the stud with two sapphire jewels secured by a screw clamp 43. An end ring 46 ground to the same diameter as the jewels but slightly below the diamond point protects the jewels from being chipped when the device is inserted into a hole to be gauged. The coils are first adjusted so that the circuits are balanced when movement of the end 54 of the lever 30 due to movement of the diamond point in following the bore under test is reflected by corresponding movements of the pointer of the armature. An adjustable stop 39 limits upward movement of the lever. A second use of the instrument is illustrated for determining the depth of a shoulder in a bore from the end, the diamond point engaging the surface of a standard cylindrical piece inserted in the bore and engaging the shoulder. Alternatively the lever in this form may be dispensed with and an armature in the form of a plate corresponding to the part 54 is mounted on a spring-pressed rod passing through the casing and provided with jewelled contacts at the end. The first form may be used for measuring the diameter of a shaft by supporting the latter in a V-block on sapphire bearing strips let into it the end of the lever contacting with the shaft opposite the strips.