Ammeter

Portable Standard Ammeter
Type: Moving Coil
No: D678439
This instrument is calibrated in Absolute Amperes. It is correct at 20°C
Certified to be in accordance with B.S. Specification No. 89, Grade Precision.
Accuracy 0.5% of maximum scale value.
Resistance of Inst Range 15.5 ohms.
Use Shunt.
Temperature correction, multiply instrument readings by: Negligible.
Resistance of leads 0.05ohms.
Date 29-9-89
Elliott Brothers (London) Ltd., Century Works, Lewisham, London, S.E.13
0.726-561

The Lewisham site of Elliott Bros (London) Ltd was established as Century Works in 1900. The factory made a range of electrical meters certainly as early as the post war years. Portable instruments are standard test instruments used in electrical and electronics work. This is a moving coil ammeter; it uses magnetic deflection, where current passing through a coil placed in the magnetic field of a permanent magnet causes the coil to move. The modern form of this instrument was developed by Edward Weston,and uses two spiral springs to provide the restoring force. The uniform air gap between the iron core and the permanent magnet poles make the deflection of the meter linearly proportional to current and as such it has linear scales. Because the magnetic field is polarised, the meter needle acts in opposite directions for each direction of current. This is a DC ammeter; sensitive to which way round it is connected so it is marked with a positive terminal.
The scale is marked for the range ±150 (presumably 'amperes' when used with the correct shunt resistance)