Patent GB-743654-A (GB)
Title: Improvements in or relating to apparatus for measuring electrical resistance
IPC: G01R27/02
Filed: 1952-11-06
Abstract: 743,654. Measuring resistance. GENERAL ELECTRIC CO., Ltd., and ASHER, P. Nov. 3, 1953 [Nov. 6, 1952; Jan. 21, 1953], Nos. 27990/52 and 1836/53. Class 37. Apparatus for measuring high-value resistance (e.g. insulation resistance) comprises two R-C circuits and means for comparing the time periods required for the capacity C in each R-C circuit to attain a predetermined voltage. The invention provides dry-battery operated selfcontained portable and megohms measuring equipment utilizing cold-cathode triodes and wherein the capacity-charging time periods are compared by counting the number of charge/discharge cycles through which one R-C circuit goes during the period that the capacity C in the other R-C circuit is being charged to the predetermined voltage. The embodiment of Fig. 1 accurately measures pure resistance or capacity-affected resistance. The R-C circuit which goes through the charge/discharge cycles is R6-C1 and the other R-C circuit comprises R as the test or a calibrating resistance and C as C2, C3 or C4 selected in accordance with the measuring range required. The electromagnetic counter M counts the charge/discharge cycles of R6-C1 and thus indicates the megohms value of the resistance under test. The charging voltage for both R-C circuits is drawn from potentiometer R1/R2. Setting up for test use.-With S2 in CHECK VOLTS position contacts F, G are closed so that depressed momentarily button P operates and locks relay RL3 and battery B2 voltage is switched on across potential divider R1/R2. Next R2 is adjusted until neon V2 through A-B closed contacts just glows and indicates charging voltage. CALIBRATION may also be checked (see below). Measuring pure resistance.-S2 is turned to TEST 1 where closed A, D contacts apply R1/R2 charging voltage to an R-C comprising the unknown R across the TEST terminals and one of C2-C4 (say C2) selected by S1. Also R1/R2 charging voltage is applied through R6 to charge C1. Whilst C2 is charging C1 charges/discharges as V3 fires to operate RL2 and magnetic counter M and discharge C1 through resistance R15 until V3 eventually deionizes through R8. When C2 is charged sufficiently V1 strikes and relay RL1 operates to open circuit RL3 and the value of the test resistance is read off M as the number of charge/discharge cycles of C1. Also with RL3 reverted C2 discharges through R7. Eventually V1 extinguishes and RL3 again operates and locks. The R6-C1 circuit includes one of pre-set resistors R5, R12, R13 selected by S1 so as to compensate for deviations of the ratios of the capacities C2-C4 from the range measuring ratios. Measuring capacity-affected resistance, i.e. a test resistance R with inherent or associated capacity C.-A test is made as above with S2 in TEST 1 position; also a measurement is effected with S2 in TEST 2 position and S1 selecting a capacity C6-C8 for connection across the resistance under test (i.e. across the TEST terminals) via closed contacts H, K. The two values obtained are apparent resistance values and true resistance is computed from a graph, Fig. 2 (not shown). The principle underlying this method of measuring true resistance R in a combination R + Cr by switching in parallel with R + Cr a selected capacity (say C6) is fully developed mathematically in the Specification. In the TEST 2 process the selected C6-C8 automatically discharges through R14. Calibration.-S2 is set to CALIBRATION and H, J contacts close to connect a known resistance R16 across the TEST terminals. If necessary R6 is adjusted until M indicates the value of R16.