a meter for measuring electrical resistance in ohms
A portable device for measuring relatively small values of electrical resistance.
An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current. Micro-ohmmeters make low resistance measurements. Megohmmeters measure large values of resistance. The unit of measurement for resistance is ohms. The first ohmmeters were based on a type of meter movement known as a 'ratiometer'. These were similar to the galvanometer type movement encountered in later instruments, but instead of hairsprings to supply a restoring force they used conducting 'ligaments' instead. These provided no net rotational force to the movement. 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.
The Standard Electrical Dictionary
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. W. E. Ayrton. 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.
An instrument or meter used to measure electrical current.
Technicians and electricians use an ohmmeter for various different purposes.
The numerical value of ohmmeter in Chaldean Numerology is: 9
The numerical value of ohmmeter in Pythagorean Numerology is: 7
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