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 functionality and accuracy of an Avometer electrical instrument.
| Identifier | ExFiles\Box 165\3\ img072 | |
| Date | 12th February 1929 | |
| EFC. AD.24. 12th February, 1929. AVOMETER ELECTRICAL INSTRUMENT. This instrument is of the moving coil type and is fitted with a turn switch providing four current ranges from .012a to 12.0a and five volt ranges from .12 v to 1200 v, also in conjunction with a dry cell housed in the case and a special scale marked to read from right to left and adjusting screws, four resistance ranges from 1000 ohms to 1 megohm. The two higher resistance ranges require a 7.5 and 75 volts supply respectively. All four resistance ranges are provided for on the turn switch which has thus thirteen working positions. There is one pair of leads attached to the instrument for all purposes. The general accuracy is good volt ranges being within +1% over the greater part of the scale, though no high readings were taken on the 1200 v range. The same applies to the .012a and .12a ranges and the bare instrument taking .006a a full scale deflection. The 1.2a and 12.0a ranges read about 2.0% low on the average. There is a rheostat on the instrument which is included in the current range circuit and is short-circuited at the low resistance end(marked "off") rising to about 460 ohms resistance all in. It is intended to carry up to 12.0a in the first section and up to 1.2a afterwards, but the labelling on the knob appears to be reversed, making it read as though 12.0a may be carried in the high resistance position and only 1.2a in the low. The resistance ranges are mainly within 1.5% of correct values in the middle portion of the scale, but the error is apt to be larger at either extreme. The instrument should prove to be of general use but it cannot be used for measuring both volts and amperes at the same time as with our Weston instruments. The movement is not as well damped as is desirable. Also great care will be needed if the instrument is constantly in use on a variety of work as with all the ranges subject to one turn switch, there is great danger of getting an excessive current through the coil and causing damage. EFC/F.J. Allen. | ||