Electric potential and electromotive force are alternate names for voltage.

Remember that voltage is present even if no current is flowing; a battery may be a 9 volt battery even though it is not connected to anything. Thus, it is referred to as electric potential, or electromotive force.

And, as you may guess, voltage is measured with a voltmeter.

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A voltmeter is used to measure the potential difference (voltage) between two points in a circuit.

The voltmeter is connected in parallel with the component or points whose voltage you want to measure; that means it is placed across the two terminals of the component so it reads the voltage drop across that element.

An ideal voltmeter has infinite resistance, so it draws negligible current and does not affect the circuit when connected in parallel. If a voltmeter were placed in series it would greatly reduce or stop current flow because of its very high resistance.

By contrast, an ammeter is connected in series with the circuit element whose current you want to measure. An ideal ammeter has zero resistance so it does not introduce a voltage drop and does not change the circuit current.

Be careful to distinguish a voltmeter (measures voltage, connect in parallel) from an ammeter (measures current, connect in series).

Memory aids:

• VoltMeter Parallel = VaMPire

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An ammeter measures current flow. Unlike voltage, which is a potential difference, current represents the actual movement of charge through the circuit. To measure how much current is flowing, the current must flow through the meter itself. Therefore a meter configured to measure current (an ammeter function on a multimeter) must be connected in series with the circuit so the same current passes through the meter.

A multimeter set to measure current ideally has very low internal resistance so it does not drop voltage or significantly change the circuit. Because of that low resistance, do not connect the meter across a power source with no load — that would be like shorting the source and can blow the meter’s fuse or damage the meter.

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Electric current is measured in amperes (amps), and is measured with an ammeter. An ammeter is connected in series with the circuit so the same current flows through it; it is designed to have a very low internal resistance so it does not significantly change the current being measured.

Other common instruments are:

• Ohmmeter: measures electrical resistance (ohms)
• Electrometer: measures very small charges or voltages and is designed for extremely high input impedance
• Voltmeter: measures electrical potential difference (volts)

Memory aids:

• ammeter → amps
• voltmeter → volts
• ohmmeter → ohms

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An ohmmeter works by sending a small known current through the component and then measuring the voltage across it.

Using Ohm’s law:

\[R = \frac{E}{I}\]

• The meter controls (I) (a small test current),
• Measures (E) (the voltage that results),
• Then calculates (R) and shows you the resistance.

So the key idea: apply a small current, measure the resulting voltage, use \(R = \frac{E}{I}\).

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A multimeter in the resistance (ohms) setting works as an ohmmeter: it applies a small test current through the component under test and measures the resulting voltage to compute resistance.

If you connect an ohmmeter to a live voltage source, that external voltage can force current into the meter's input circuitry in a way it was not designed to handle, possibly damaging components. For this reason you should always verify the meter is in the correct mode before connecting it to a circuit.

As a general safety practice, start on the highest voltage range when measuring an unknown voltage and work down to prevent overrange or damage. Also, never connect a meter set to measure current (ammeter mode) directly across a voltage source — that effectively shorts the source and can blow the meter's internal fuse or cause more serious damage.

Memory aids / quick reminders:

• Never try to measure voltage while the meter is set to the resistance (ohms) mode.
• Always check the meter mode before connecting to a circuit; when unsure, start at the highest voltage range.
• Never connect a meter in current (ammeter) mode directly across a voltage source — it will short the circuit and likely damage the meter.

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A multimeter is named because it combines several measurement functions in one device. Most common multimeters provide an ohmmeter (for resistance), a voltmeter (for voltage), and an ammeter (for current). That is why a multimeter is used to measure voltage and resistance. Measurements such as signal strength and noise are made with RF or audio test equipment, and impedance/reactance of components are typically measured with an LCR meter or an impedance analyzer, not a simple multimeter.

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Acid-core solder is used to strip the oxidation layers off of metal objects to create a joint. It is mostly used in applications like plumbing where a water-tight seal is needed. It is not suitable for electronics due to the corrosive effects — the acidic flux residue can corrode thin metal contacts and cause circuit failures over time.

• Tip: Acid would kill the sold(i)er

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A "cold" solder joint happens when the parts being joined are not heated enough, the flux hasn't properly prepared the metal surfaces, or the molten solder is disturbed while it is cooling through its plastic phase. When that occurs the solder doesn't flow and wet the surfaces smoothly. Instead of a smooth, bright, shiny fillet, the joint will look dull, grainy, rough or lumpy and may have small cracks where it didn't adhere properly. Such joints are mechanically weak and can cause intermittent or failed electrical connections. To avoid them, heat the part (not just the solder), allow the solder to flow fully, then remove the iron and let the joint solidify undisturbed.

Memory aids / quick tips:

• A good solder joint looks bright and shiny; a cold joint looks dull, rough, or lumpy.
• Heat the work, not just the solder; let solder flow and then stop touching it until it cools.
• Disturbing solder during the "plastic phase" produces a grainy, weakened joint.

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An ohmmeter applies a small DC voltage and measures the resulting current to determine resistance. When it is connected across a discharged capacitor, the meter’s voltage causes the capacitor to charge. At the moment of connection the capacitor looks like a short (so current is relatively high), but as it charges the charging current falls exponentially toward zero. Because the measured current decreases while the applied voltage stays the same, the meter interprets this as an increasing resistance. Eventually, once the capacitor is fully charged, the meter will read a very high (near-infinite) resistance.

Memory aids:

• Capacitor charging → current falls → resistance reading goes up
• Think “charging = slowing current = rising resistance”

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An ohmmeter measures resistance, not voltage. Since resistance is the ratio of voltage drop to current through a portion of a circuit (R = E/I), the ohmmeter applies a small test current to the component or circuit and measures the resulting voltage drop.

If the circuit is powered while making the measurement, the external voltage will affect the meter's readings and can be much higher than the ohmmeter's test voltage. That can give incorrect resistance readings and may damage the ohmmeter.

• Always ensure the circuit is unpowered before measuring resistance with an ohmmeter.

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