Patent GB-200977-A (GB)
Title: Improvements in or relating to electrical measuring instruments
IPC: G01R13/00
Filed: 1922-05-20
Abstract: 200,977. Macadie, D. May 20, 1922. Combinations of instruments; resistance, cur rent, and pressure meters; connections, varying; testing instruments.-A combined portable electric measuring apparatus is arranged to read current, voltage and resistance on a single moving-coil instrument. Resistances, a battery, and a switch may be arranged in connection with the instrument so that, by means of the switch and without altering the testing leads, the circuits for such measurements may be changed and the sensitiveness of the instrument when used as an ammeter of voltmeter may be varied by shunts or series resistances respectively. The moving- coil instrument a may be shunted by a resistance b approximately to the resistance of the moving coil and comprising wire of the same resistance temperature coefficient as the moving coil and by a variable resistance c, one terminal of the instrument a being connected to an external test terminal l. A switch arm m is retained in contact with one of studs 1 - - 7 by a notched segment m<2> co-operating with spring blades m<3>, m<4>, the blade m<3> being connected to a second external test terminal k and the blades m<3>, m<4> being connected together through a variable resistance g and through external terminals h, j normally connected together. Studs 1 - - 3 are connected to tappings on the resistance b so that the instrument a measures current in a circuit connected to terminals k, l in hundredths or tenths of amperes or amperes respectively. The current during such measurements may be adjusted by the resistance g. Resistances e, fare connected between the studs 4, 5 and the instrument a as shown, so that the latter measures a voltage applied to terminals k, l in volts naultiplied by ten or volts according as arm m is on stud 4 or stud 5. The current and voltage readings are all read on one scale on the instrument a. In cases where the apparatus under test is not connected to a current source, the normal connection between terminals h, j may be broken and replaced by an outside current source, leaving terminals k, l free for connection to the apparatus. Stud 7 is connected through a resistance o, a variable resistance d and a dry battery p to the contact arm of the resistance c, the instrument a measuring on a second scale a resistance between terminals k, l when arm m is in stud 7. In order to compensate for changes in the voltage and internal resistance of the battery p, terminals k, l are short-circuited and the resistance d is adjusted until instrument a gives zero reading with the arm m on stud 7. The arm m is then placed on stud 6 which is connected to stud 7 through a resistance n of 200 ohms, and, if the instrument a does not read 200 ohms, the resistance c is adjusted until the reading of the instrument a has altered to an equivalent extent on the other side of the 200 ohms mark and the resistance d is then adjusted to bring the reading back to 200 ohms. The whole apparatus is enclosed in a portable box. Variable resistances; wire resistances, cored.- Resistances c, d, g may comprise insulated wire wound on an insulated copper or other metal strip afterwards bent to shape, the strip dissipating heat and the wire being bared at one edge of the strip for contact with the rotatable arm. The first part of the resistance g is of stout wire sufficient to carry the maximum current used. The resistance b is wound on a straight flat vulcanite strip.