The measure of how much an object (electrical connections such as
joints, terminations, connectors) opposes allowing the electrical
current to pass through it, is called electrical resistance. A test
is performed to measure this resistance and this test is commonly
known as the Ductor test. The instrument used to perform the ductor
test is called an Ohmmeter, and since it's used to perform the ductor
test, the ohmmeter is also known as a ductor tester.
As an electronic instrument that checks the complete circuit or
measures the resistance of a single circuit element, the
ductor
tester can be found in many variations as Micro, Mega and Milli-
Ohmmeters, which are used to measure resistance in different
applications of electrical testing. The ductor tester consists of a
DC ammeter and a few other things such as:
- A 3V battery which is a DC source of potential;
- Resistors; one or more of which at least one is variable.
Before the test starts, the test leads of the
ductor tester are
shorted together. Then, the deflection of the needle, which shows the
result of the measuring, is controlled by the amount of battery
current. The meter is adjusted for operating at the selected range
and the needle drives back at the maximum position on the Ohms scale,
and the meter current is maximum. After using the ductor tester, the
test leads should be removed, because if they stay on, the battery of
the meter gets discharged. Afterwards, when the rheostat is adjusted
and the test leads shorted, the needle comes to zero position and
this means zero resistance between the leads.
Generally, there are two types of ductor tester:
- Series Type Ohmmeter
Here you have 4 resistors (R1 -current limiting resistor, R2- zero
adjust resistor, RX-unknown resistor and RM-internal resistor), an
internal battery voltage - E and output terminals, A and B. If you
connect the A and B terminals with the R1 and R2 resistors the
battery will form a simple series circuit. If the two terminals are
open circuited, no current will flow and the needle won't move, and
this indicates infinite resistance.
- Shunt Type Ohmmeter
This ductor tester is used for measuring small values of current
resistance. The movement mechanism is connected parallel to the RX,
and in this ductor tester it's compulsory to use a switch; if you
don't, the current will always flow in the movement mechanism. In
this instrument, when the A and B terminals are closed, the needle
reads zero because the current flows only through the resistor RX.
When the two terminals are opened, there is no current flowing
through the RX resistor so the reading on the ductor tester is marked
as infinite.
very comprehensive article.
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