Patent GB-404015-A (GB)
Title: Improvements in and relating to multi-range electrical measuring instruments
IPC: G01R15/08
Filed: 1932-07-06
Abstract: 404,015. Current and pressure meters ; resistance measurements. MACADIE, H. S., and AUTOMATIC COIL WINDER & ELECTRICAL EQUIPMENT CO., Ltd., Winder House, Douglas Street, London. July 6, 1932, No. 19126. [Class 37.] A multi-range alternating-current measuring instrument comprises a single moving-coil direct-current meter connected to the output of a rectifier fed from a transformer, the current or voltage to be measured being applied to tappings on the primary winding of the transformer, or on a resistance in series with the whole or part of the primary winding, so arranged that for any given proportion of the full scale value on any given range the secondary output current fed to the rectifier is constant, whereby the same scale shape is obtained on all ranges. A moving coil meter d is connected across a bridge-arranged fullwave metal rectifier e connected to the secondary winding f of a transformer f <1>. One end of the secondary winding f is connected to one end of the primary winding f and to one input terminal c, the other input terminal b being connected to the arm a<8> of a range-changing switch having contacts a<1>-a<7> each corresponding to one range. Contacts a<5>, a<6>, a<7> are connected to tappings on the primary winding f<2>, the terminals a<3>, a<4> being connected to a tapping through resistances j, j<3>. In series with the primary winding f is a high resistance j<l> connected to the contact a<1> and having a tapping j<2> connected to the contact a<2>. Contacts a<1>-a<4> are used for voltage measurements, contacts a<5>-a<7> being employed for current ranges. A resistance g, connected between the rectifier e and one end of the secondary winding f is in circuit when measurements of low voltages are being made with the switch arm a<8> on the contact a<4> so as to counteract the non-linear characteristic of the rectifier at low voltages, and at other times is short circuited by a switch h. For the low voltage ranges a step-up transformer ratio is employed to feed the rectifier. The instrument may also be provided with a second range-changing switch enabling direct currents and voltages and also resistances to be measured by the same meter as is employed for the alternating current measurements. A cam on the alternating current range switch is arranged so that in one position of the switch, marked " D.C. readings," a multiple-blade switch disconnects the rectifier from the transformer, disconnects the moving coil meter from the rectifier, and connects the resistance in series with the primary winding of the transformer in series with the moving coil meter. A second cam-operated switch inserts the resistance g when the switch is in position for low-voltage alternating current readings. The direct-current range switch has points inserting the series or shunt resistances required for the different ranges of voltage and current and also points enabling resistances to be read, these arrangements being similar to those described in Specification 200,977, [Class 37, Electricity, Measuring &c.]. An additional contact, marked " A.C. readings " connects the instrument terminals to the A.C. range switch.