Electrical measurements

Voltage sources

Batteries and other voltage sources have internal resistance.  When they do work moving charges against the electric force, some of this work is already converted to thermal energy in the battery.  The amount of energy lost to thermal energy in the battery is Ir, where I is the current flowing in the circuit and r is the internal resistance of the battery.  The voltage across the battery terminals therefore drops from the nominal value V to (V - Ir) when a current is flowing in the circuit.  In a circuit diagram we represent the internal resistance of the battery by a resistor r connected in series with the emf.

Electrical Measurements

A voltmeter is a device used to measure voltages, while an ammeter measures currents.  Meters are either analog or digital devices.  Analog meters show the output on a scale with a needle, while digital devices produce a digital readout.  Analog voltmeters and ammeters are both based on a device called a galvanometer.  Digital voltmeters and ammeters generally determine the voltage drop across a known resistor and then convert the result to a digital value for display.

Voltmeters

Resistors in parallel have the same voltage across them.  If you want to measure the voltage across a circuit element, such as a resistor, you place the voltmeter in parallel with the resistor.  The voltmeter is shown in a circuit diagram as a V in a circle, and it acts as another resistor.  To prevent the voltmeter from changing the current in the circuit (and therefore the voltage across the resistor), the voltmeter must have a resistance much larger than that of the resistor.  If the resistance of the voltmeter is large, only a negligible current flows through the meter.

Ammeters

Resistors in series have the same current flowing through them.  An ammeter, must be placed in series with a resistor to measure the current through the resistor.  On a circuit diagram, an ammeter is shown as an A in a circle.  The ammeter acts as a resistor.  To prevent the ammeter from changing the current in the circuit, the ammeter must have a very small resistance compared to the resistance R of the circuit.

Ohmmeter

An ohmmeter uses an internal battery to send a known current through the resistor.  The ohmmeter then measures the voltage across the resistor, and displays the resistance R = V/I.  The resistor must be disconnected from the rest of the circuit.

Electrical safety

Consider a standard electrical outlet.  The transformer coil, hot wire, appliance, and neutral wire form the circuit. The neutral return wire is grounded at various places. The ground wire is a separate path to the ground.

If the electrical equipment housed in a non- conducting case, then the user is protected, even if a fault inside the case causes the hot wire to touch the case. But if the equipment is housed in a metallic case, then such a fault can give a severe shock to the user if the case is not independently grounded. The user becomes a lowest-resistance path to the ground, and a large current will flow through the user before (hopefully) the fuse blows.

To prevent such a shock the case must be connected directly to the ground.  Then the lowest-resistance path to the ground is through the case and the user is protected.  Equipment with a metal case either uses a three-prong plug or a polarized two-prong plug. The prong that makes the connection with the neutral wire is larger, so that the plug can only inserted one way into the outlet (without forcing it in).

For equipment with a three prong plug, the case is connected directly to the ground through the ground wire.  For equipment with a two prong plug, the case is connected directly to the ground through the neutral wire.

 The ground wire is tied to the case. The neutral wire is tied to the case.

Do not use cheater plugs to plug three prong plugs into two prong extension cords!  While faults are not likely, they can happen.  Do not risk it!