GB654168A - Improvements in or relating to indicating devices - Google Patents

Abstract

654,168. Electric speed indicators. SMITH & SONS (ENGLAND), Ltd., S. Nov. 14, 1947, No. 30350. [Class 40 (i)] An electric indicating device comprises an indicator responsive to electric signals representing a variable, in circuit with a relay which is also responsive to the signals to modify the indicator circuit and thereby alter the range of magnitudes of the variable to which the indicator is responsive. In an electrical speedometer, an A.C. or D.C. generator 10 (Fig. 1), is driven in accordance with the speed, e.g. of a locomotive and its output varies accordingly. A current or voltage measuring instrument 12 indicates the speed and is connected in series with the generator through a resistance 13 normally shorted out by the contacts of relay 17 connected across the generator. At low speeds the instrument 12 registers in its most sensitive condition but at a speed corresponding to its full scale deflection, relay 17 operates and inserts series resistance 13 so that the instrument indicates speed on a second range with a higher maximum reading. In a modification (Fig. 2 not shown), a first generator is connected to the indicator at low speeds until full scale deflection is reached, when a relay is operated by the second generator (which has a lower output than the first), to connect the second generator to the indicator, instead of the first, giving a second speed range with a higher maximum reading.