Re: What is an ohmeter?

In answer to your questions, let me first explain how an meter movement 
and ohmmeter works (and even then, there are several ways to measure 
resistance).

From your question, I am assuming that you are using an analog meter.  If 
you are using a digital one, they work a little differently (solid state 
digital measurement electronics have different voltage chracteristics than 
analog ones).

For analog measurement devices, a d'Arsonval meter movement is used.  This 
movement consists of a movabe coil placed in the field of a permanent 
magnet. When applied, current in the coil creates a torque on the coil, 
which then rotates until this torque is exactly balanced by a restoring 
spring.  As the coil rotates, it moves a pointer across a calibrated scale 
(for volts, amps or ohms).  This movement is designed so that the 
deflection of the pointer is directly proportional to the current in the 
movable coil.  These meter movements come in different voltage and current 
ratings - several commercial movements can be found with 50mV and 1 mA, 
100mV and 10mA, and 1v and 500mA ratings.  These ratings are important: 
when a coil is carrying its rated current, the voltage drop across the 
coil is the rated coil voltage and the pointer is deflected to its full-
scale position.  The current and voltage rating of the coil also specify 
the resistance of the coil.  Thus a 50mV, 1mA meter movement has a 
resistance of 50 Ohms.  These movements can be used as ammeters, 
voltmeters, ohmmeters and nul detectors.  It all depends on how you apply 
Ohm's law (v=iR where v=voltage, i=current and R=resistance) and 
Kirchhoff's circuit laws (the sum of all voltages in a circuit equal zero 
and the sume of all currents in a circuit equal zero).

To measure a resistance with a meter movement, this is typically done two 
ways (by analog means).  The first method is to use an ohmmeter circuit.  
This consists of a meter movement and a battery in series with a 
regulating resistor (typically a variable resistor).  To use the ohmmeter 
circuit, you first short  the terminals of the circuit (meter movement, 
battery and regulating resistor in series)  and use the variable resistor 
to calibrate the meter so that at a short circuit the meter gives maximum 
deflection.  This is then labeled as zero resistance.  When you apply an 
unknown resistance to the circuit terminals, the deflection will be less 
than full scale.   You then can calibrate a scale (from right to left) 
usng a series of known resistors.  This circuit isn't without it's 
limitations - the resistance scale is inherently non-uniform and the scale 
can get quickly cramped at the high-resistance end of the scale.  While 
this particular circuit is not a precision instrument, it is a very simple 
one to use.  This is usually the circuit implemented to check the 
continuity of a circuit (to determine whether R is less than infinity)

Here is what the circuit would look like where *=wire, V=DC Voltage 
Supply, M=Meter, Rb= battery resistance, R=Variable Resistor and 
Rx=unknown resistance:

*************M**************
*                          *
Rb                         *
*                          *
+                          Rx
V                          *
-                          *
*                          *
*************R**************

The reason this has to be in series is because of the battery resistance.  
This is typically small, and the total regulating resistance would be 
Rb+R.  If it were in parallel, the regulating resistance would be (Rb*R)/
(Rb+R).  Depending on the accuracy required you may need to account for 
the Meter resistance as well, thus complicating the resistance calculation 
further.  The regulating (variable) resistor is used to compensate for 
inherent changes in the internal resistance for the battery (it may change 
with temperature and charge values over time). 

A much more precise measurement circuit is called the Wheatstone bridge.  
It is good for measuring medium resistor values in the range of 1 Ohm to 
10 MegaOhms.  Commercial models of the wheatstone bridge have measurement 
accuracies on the order of +/- .05%.  The bridge consists of four 
resistance branches, a dc voltage source and a d'Arsonval movement in the 
microAmp range. The circuit looks like this (where * denotes wires, R are 
resistor values and M denotes the meter):

*********************
*                  * *
*                 R1  R2
+                *      *
V               ****M*****
-                *      *
*                 R3   Rx
*                   * *
**********************

Rx is the resistance you're trying to find.  The value for Rx is 
calculated using Ohm's law: 
Rx=(R2/R1)*R3

The resistances for R1 and R2 must be exactly the same (as close as 
possible) so that the calue of their relationship is 1.  This then leaves 
the equation of Rx=R3 where Rx is the unknown resistance and R3 is a very 
accurate variable resistor.  The reason for the meter is that if there is 
any detectable current in the meter, the circuit is out of balance and R3 
must be adjusted to compensate.  When the current in the meter equals 
zero, you then have the measured resistance value for Rx.  This circuit is 
not very accurate for measuring very low resistances because the of the 
thermo-electric voltages generated at the junctions of dissimiliar metals 
(the thermocouple effect) and because of thermal heating effects on the 
resistors themselves.  Very high resistances are also difficult to measure 
because of current leakage effects in the electrical insulation of the 
resistors themselves.

I hope this answers your questions and gives you some insight on how an 
ohmmeter works and how to apply it to measuring resistance.