Patent GB-602425-A (GB)
Title: Improvements in or relating to electrical apparatus employing bridge circuits
IPC: G01L1/22
Filed: 1945-10-20
Abstract: 602,425. Resistance measurements ; determining dimensions of materials. CLARK, E. F. Oct. 20, 1945, No. 27643. [Class 37] In sensitive bridge apparatus in which a battery or other source of supply and a galvanometer or other indicator are connected with the bridge proper by leads each including a slip contact, control or other auxiliary means are connected to the bridge proper by independent slip contacts additional to those carrying the supply and galvanometer currents. In the apparatus shown, an assembly of six coaxial spaced insulated slip rings 1a, 2a, 3a, 3b, 2b, 1 b is arranged to rotate with a body, such as an airscrew, to be tested for radial or torsional strain. The bridge proper comprises two pairs of similar strain gauge elements 11a, 11b and 12a, 12b secured by adhesive to the surface of the body under test. The junctions 11c, 12c of these pairs are connected to the inner rings 3a, 3b. The elements 11b, 12a are sensitive to radial strain, the other elements 11a, 12b being sensitive to transverse strain. The outer ends of the elements 11a, 12a, 11b, 12b are connected to the rings 1a, 1b. The rings 2a, 2b serve as, guard rings and may be connected to the midpoint of a resistance 4 connected between the outer rings 1a, 1b so as to rotate with the assembly. Alternatively, the guard rings 2a, 2b may have single brushes 2c, 2d which are connected to the midpoint of a resistance 5i which is connected between supply leads 6a, 6b from a battery 7 to supply brushes 1c, 1d of the rings 1a, 1b. Two further brushes 1e, 1f. engaging the supply rings 1a, 1b form individual slip contacts connecting a voltmeter V with the supply points of the bridge proper, whilst third brushes 1g, 1h on the rings 1a,. 1b provide individual slip contacts for connecting one end of an adjustable balancing shunt 8 with the bridge. The indicator rings 3a, 3b have their brushes 3c, 3d connected to a galvanometer G and one of them, 3b, has an additional brush 3f connected to one end of the shunt 8, the other end of which is connectable through a two-way switch 9 and brushes 1g, 1h with either supply point of the bridge. Alternatively, both rings 3a, 3b may have second brushes connected with the two ways of the switch 9, one only of the rings 1a, 1b then having a brush connected to the resistance 8. The switch 9 enables the shunt 8 to be switched from one element 11a to an adjacent element 11b as necessary for change of sign of the balancing effect. Where more than a single bridge is required, more strain gauge elements 13a, 13b, 14a, 14b, 15a, 15b may be provided. The junctions 13c, 14c, 15c are provided with sockets for reception of plugs on leads from the rings 3a, 3b,according to which two pairs of elements are to be used at a time. A pair of elements such as 14a, 14b not in use in the bridge may be utilized in place of the resistance 4. A switch 10 enables one terminal of the galvanometer G to be connected to earth to enable the state of the insulation to be observed on the galvanometer. With the battery 7 disconnected, the bridge elements and the metal of a body under test function as electrodes and the adhesive medium and moisture as the electrolyte of a cell, giving a deflection dependent on the insulation of the system.