Also, the movement was wound with two coils. One was connected via a series resistor to the battery supply. The second was connected to the same battery supply via a second resistor and the resistor under test. The indication on the meter was proportional to the ratio of the currents through the two coils. This ratio was determined by the magnitude of the resistor under test.
The advantages of this arrangement were twofold. First, the indication of the resistance was completely independent of the battery voltage and no zero adjustment was required. Second, although the resistance scale was non linear, the scale remained correct over the full deflection range. By interchanging the two coils a second range was provided. This scale was reversed compared to the first.
A feature of this type of instrument was that it would continue to indicate a random resistance value once the test leads were disconnected. Ohmmeters of this type only ever measured resistance as they could not easily be incorporated into a multimeter design. Insulation testers that relied on a hand cranked generator operated on the same principle. This ensured that the indication was wholly independent of the voltage actually produced.
An instrument for measuring directly the resistance of a conductor or of any part of a circuit through which a strong current is passing. It is the invention of Prof. It contains two fixed coils at right angles to each other acting on the same needle of soft iron. One coil is of thick wire and is placed in series with the resistance to be measured. The other is of very thin wire and is placed in parallel with the same resistance. One wire acts by the total current, the other by the potential difference between the ends of the resistance.
The action on the soft iron needle is due to the ratio of potential difference to total currents, or to the resistance itself. By properly designing and proportioning the coils the angular deflections of the needle are made proportional to the resistance. In use the thick wire may be kept permanently in circuit.
On connecting the binding posts of the thin wire coil to any two parts of the circuit its resistance is at once given by the deflection of the needle. When no current is passing the needle rests in any position. A current in the thick coil brings it to zero. A current simultaneously passing through the thin high resistance coil brings about the deflection. The instrument is a commercial rather than a scientific one.
JavaScript seems to be disabled in your browser. For the best experience on our site, be sure to turn on Javascript in your browser. We use cookies to make your experience better. By using this website you agree to our use of cookies outlined in our Privacy Policy. Learn more. Assuming a constant anode supply voltage on the two triodes therefore, it will be apparent that there will be a difference in the plate currents of the two tubes, depending upon the degree of unbalance of the bridge circuit.
By connecting a meter between the anodes of the two tubes, this meter will indicate the difference in anode voltage between the two. This difference voltage will of course vary with the degree of unbalance of the bridge. The meter, therefore, can be calibrated on a linear scale to read the exact quantity in ohms of the resistance of the unknown leg of the bridge. Theory of Operation With the two upper arms A of the bridge equal in resistance, it will be apparent that the bridge will be balanced when the unknown resistance X is equal to the known resistance B.
Whenever X does not equal B, however, the bridge becomes unbalanced and produces an output voltage which, if the unknown resistance is to be measured on a linear scale, must vary in direct proportion to the value of the unknown resistance.
The output voltage of the bridge Bout may be expressed in terms of the input voltage Ein, A, B, and X, by the following equation:. If the above equation has its numerator and denominator divided by B2, Equation l becames stance there are ratios rather than absolute values.
It will be apparent that if the ratio is made of the order of times larger than the ratio Eout:. Unfortunately, the output voltage Bout cannot be measured directly by connecting a meter across the output terminals of the bridge because of the loading effect that a meter would have which would destroy the linearity of Equation 3.
To get around this diculty, therefore, the output voltage is connected directly between the grid and cathode of the amplifier circuit. As thus connected, the bridge output will be effectively connected to an infinite impedance. This is so, because the direct coupling to the input of the amplifier will be a substantially pure capacitance which is an infinite impedance as far as D. The meter may conveniently be a zero center scale microammeter with a suitable internal multiplier. When the plate voltages of the two triodes are equal, therefore, the meter will stand at its center position corresponding to ,zero current flow through the meter and this point, of course, will be the midpoint on the resistance scale.
Since the triode 15 has a constant grid bias and a constant plate voltage, its anode current will be constant as will its anode voltage. The anode current, and therefore the anode voltage of the triode 14, however, will vary in accordance with the degree of unbalance of the bridge, which is either added to or subtracted from the grid bias to effect the ultimate grid voltage of the triode The anode load resistors and 21 are made equal.
As one working embodiment of the above described invention, the following electrical values of the various circuit components have been found to yield satisfactory results:. Potentiometer 13 ohms 25K Potentiometer 22 do 5K Resistors 20 and 21 do 6. Both are primarily for the purpose of measuring a change in resistance, as, for example, in medical psychology work to measure a patients skin resistance under normal and abnormal conditions.
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