- Technical Blog From My Notebook

Sunday, June 19, 2016

Many measuring devices work because electric and magnetic fields produce forces proportional to the intensity of the field. By using a tension spring against which the
electric or magnetic force can pull or push, a movable needle can be constructed. The needle can then be placed in front of a calibrated scale, allowing a direct reading of thequantity to be measured. These meters work by means of electromagnetic deflection or electrostatic deflection.
Sometimes, electric current is measured by the extent of heat it produces in a resistance.
Such meters work by thermal heating principles.Some meters work by means of small motors whose speed depends on the measured quantity. The rotation rate, or the number of rotations in a given time, can be measured or counted. These are forms of rate meters. Still other kinds of meters actually count electronic pulses, sometimes in thousands,millions or billions. These are electronic counters. There are also various other metering methods.

Electromagnetic deflection

Early experimenters with electricity and magnetism noticed that an electric current produces a magnetic field. This discovery was probably an accident, but it was an accident that, given the curiosity of the scientist, was bound to happen. When a magnetic
compass is placed near a wire carrying a direct electric current, the compass doesn’t point toward magnetic north. The needle is displaced. The extent of the error depends on how close the compass is brought to the wire, and also on how much current the wire is carrying.
Scientific experimenters are like children. They like to play around with things. Most likely, when this effect was first observed, the scientist tried different arrangements to see how much the compass needle could be displaced, and how small a current could be detected. An attempt was made to obtain the greatest possible current-detecting sensitivity. Wrapping the wire in a coil around the compass resulted in a device that would indicate a tiny electric current (Fig.). This effect is known as galvanism, and the meter so devised was called a galvanometer.

Once this device was made, the scientist saw that the extent of the needle displacement increased with increasing current. Aha—a device for measuring current! Then, the only challenge was to calibrate the galvanometer somehow, and to set up some kind of standard so that a universal meter could be engineered.