Potentiometers and galvanometers
Potentiometers
Once you have standards, you can make use simple potentiometer or bridge circuits to compare them with unknowns, by dividing either the reference or the unknown until a ratio is found that makes one equal to the other. In fact this technique is a very useful one because it helps to overcome a common problem in science and engineering, that of making measurements without influencing the circuit under investigation. Many kinds of resistance bridge and potentiometer were made over the years with this type of application in mind.
Tinsley potentiometer panel
Potentiometer switches
Inside Tinsley potentiometer
This Tinsley instrument uses an external galvanometer to indicate the null, achieved when a voltage having been standardised, is divided by the correct ratio to equal the unknown using the resistance switches on which a value can be selected to four significant figures of precision. The painstaking care with which equipment of this type was handmade is remarkable by today's standards.
Potentiometer knob
BIC potentiometer
Potentiometer resistances
Galvanometers
PO galvanometer 154
Kershaw Q and I detector
Early galvanometers were basically descendants of the early experiment whereby a magnetised needle such as a compass needle hung on a thread was found to deflect when current was passed through a nearby wire due to the magentic field produced by the current. A pivoted needle passing through a coil served to indicate approximately the magnitude and direction of a current flowing, with enough sensitivity to detect the null of a bridge or examine the operation of a telegraph circuit for example. The Q & I galvanometer or Linesman’s detector, designed to make measurements of both Quantity and Intensity (in effect current and voltage) used two coils of low and high resistance respectively. They were sufficient for making comparisons, but not precise measurements.
Moving coil instruments
Moving-coil galvanometers were the first current-indicating devices that could be made linear enough and sensitive enough to make direct measurements of unknowns to a high degree of accuracy, relying on their own calibration and their minimal influence on the measured quantity. Like many kinds of instrument they work by establishing an equilibrium. The current to be measured, or a known fraction of it, passes through a rotatable coil inside a magnetic field. A torque is exerted on the coil that works against the restoring force of a spring, to determine the position of the pointer. The long scales of laboratory-grade galvos allow precise readings to be taken by eye, a factor as important as the underlying accuracy. In the lab or factory, a bench galvo could be used as a sub-standard, for example, calibrated periodically against a standard that is less prone to drift or wear. By the addition of shunts and multipler resistances, the moving coil instrument can be turned into that most versatile measuring device, the multimeter.
Heayberd milliammeter
Scalamp galvo
Cambridge milliammeter
Electronic meters
With the advent of the thermionic valve, the first effective electronic amplifying technology, came the possibility of increasing the sensitivity and input impedance of electrical instruments by adding valve stages to their inputs. The characteristics of a valve when used in a simple circuit without feedback are not inherently stable enough to maintain the accuracy required of a precision instrument, therefore early valve voltmeters required standardisation immediately before use. The Moullin Pattern ‘A’ valve voltmeter below dates from the 1920s; for comparison, the CT471 - a transistorised analogue multimeter from the 1970's - is also shown below.
Cambridge valve voltmeter
Inside Cambridge valve voltmeter
Inside CT471 multimeter
CT471 multimeter
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