From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
The resistance characteristics and calibration of a Weston instrument.
| Identifier | ExFiles\Box 34\2\ Scan055 | |
| Date | 17th October 1922 | |
| Contd. -2- EFG1/T17.10.22. RESISTANCE. The resistance is divided into two concentrically wound parts, connected in series. The outer coil is a single layer and adjustable by a sliding contact. The values of the resistances are :- Inner resistance 200.0 ohms. Outer, adjustable from 0 to 64.2 ohms. Total range therefore 200 to 264.2 ohms. A suitable adjustment between these limits enables the speed indication on the Weston instrument to be set exactly correct at any point of the scale. The resistance is of Manganin wire of small temperature coefficient, of size 38 S.W.G. the insulation of the non-adjustable portion being D.S.C. Weston Instrument. This is calibrated in steps of 50 r.p.m. up to 2000 r.p.m., and has the following characteristics. Volts for full scale deflection .492. Amperes " " " .0141. The resistance of the instrument is therefore 34.9 ohms. The adjustment of the resistance to enable the instrument to read correctly at a speed of 1000 r.p.m. necessitated the inclusion in the circuit of 30 ohms of the available 64.2, so that the total resistance of the circuit was 200 + 30 + 34.9 + 8.4 = 273.3 ohms. It was found that the electrical proportionality of the instrument was very good so that the operating voltage on the instrument at 1000 r.p.m. would be .2485, and since this voltage corresponds to a resistance of 34.9 ohms, the total E.M.F. of the machine would be 273.3 ----- x .2485 = 1.945 volts. 34.9 It was found that lower and higher speed readings were in error according to the following table:- Reading. Speed. Ratio. % correction. 500 495 .990 -1.0 790 795 .994 - .6 1000 1000 1.0 0 1250 1285 1.028 +2.8 1500 1580 1.052 +5.2 2000 2150 1.075 +7.5 | ||