Galvanometer

Part of Basic Electrical Circuits

How to build and use a galvanometer—the foundational electrical measuring instrument—using wire, a magnet, and basic metalworking.

Why This Matters

Without instruments, electrical work is blind. You cannot see current flowing, cannot tell whether a circuit is live, cannot compare resistances, and cannot diagnose faults without eventually touching something you should not. The galvanometer—a device that detects and measures small electric currents—was the first practical electrical measuring instrument, and it remains the core of every analog meter: voltmeters, ammeters, ohmmeters, and multimeters all begin with a galvanometer movement.

A galvanometer can be built with a coil of wire, a compass needle or small magnet, a cardboard or wooden frame, and a pointer fashioned from a light wire or straw. With these materials, a rebuilding community can test circuits, identify faults, measure relative voltages and currents, and verify that generators and batteries are producing output. This is the difference between working with electrical systems intelligently and working blindly.

The galvanometer was independently discovered by Johann Schweigger and André-Marie Ampère in 1820, within weeks of Oersted’s discovery that current deflects a compass needle. Within decades it had become the indispensable laboratory and telegraph instrument. A community that masters its construction gains access to the same diagnostic capability.

The Physics: How a Galvanometer Works

A current-carrying conductor in a magnetic field experiences a force. This is the fundamental principle behind both motors and galvanometers.

In a galvanometer, a coil of wire is suspended between the poles of a permanent magnet. When current flows through the coil, the magnetic force twists the coil against a restoring spring. The deflection is proportional to the current—more current, more twist. A pointer attached to the coil indicates the deflection on a calibrated scale.

Key factors that increase sensitivity:

• More turns in the coil (multiplies the force)
• Stronger magnet (increases force per unit current)
• Lighter, more freely suspended coil (reduces friction)
• Weaker restoring spring (deflects more per unit force)
• Longer pointer arm (amplifies deflection at the scale)

Building a Simple Tangent Galvanometer

The tangent galvanometer is the simplest type and the easiest to calibrate—it uses a compass needle directly.

Materials:

• A compass or compass needle (magnetized steel needle suspended on a pivot)
• Insulated copper wire (0.2–0.5 mm diameter), at least 10 meters
• A non-conductive former: cardboard tube, wooden ring, or carved bone ring, approximately 10–15 cm diameter
• Two binding posts or screw terminals

Construction:

1. Wind 10–20 turns of insulated wire around the ring former. Wind all turns in the same direction.
2. Attach the wire ends to two binding posts. This is the coil.
3. Mount the coil vertically. Orient it so that the plane of the coil aligns with magnetic north (the coil is parallel to the compass needle when no current flows).
4. Place the compass at the center of the coil with the needle free to rotate in the horizontal plane.
5. The pointer needle is initially at zero—pointing north, parallel to the coil plane.

Operation: When current flows through the coil, the magnetic field it creates acts at right angles to the coil plane. The compass needle deflects, seeking a balance between the Earth’s magnetic field (pointing north) and the coil’s field (pointing east-west).

The current is proportional to the tangent of the deflection angle: I = (H × 2πr × tan θ) / (μ₀ × N)

In practice, you don’t need this formula. Just note that deflection angle increases with current—calibrate by comparison with a known current.

Building a D’Arsonval Movement

For a more sensitive and practical instrument, the D’Arsonval movement uses a coil that rotates between fixed magnet poles. This is the basis of virtually all analog meters.

Materials:

• Strong permanent magnet (horseshoe or bar magnet salvaged from motors, speakers, or industrial equipment)
• Soft iron cylinder for the coil core (provides uniform magnetic field)
• Fine insulated copper wire (0.1–0.2 mm)
• Lightweight aluminum or bamboo frame for the coil
• Two fine steel springs for suspension and current introduction
• Light pointer (thin straw or aluminum strip)

Construction (simplified field version):

1. Shape a cylindrical core from soft iron, sized to fit within the magnet gap
2. Glue or bind a rectangular coil form (5 cm × 3 cm) around the core, leaving space to rotate
3. Wind 50–200 turns of fine wire on the form. Insulate each layer with thin paper or shellac.
4. Attach the coil to a central pivot—a needle pivot or stretched wire suspension works best
5. Connect fine phosphor bronze or steel springs as both mechanical return springs and electrical connections to the coil
6. Attach a lightweight pointer
7. Mount the assembly between the magnet poles with minimal clearance

Calibration marks: Connect the finished galvanometer to a battery through a high-value known resistor. Calculate the current using Ohm’s Law. Mark this deflection. Vary the resistor to mark additional points. The scale between points is approximately linear.

Converting a Galvanometer to a Voltmeter

Add a large series resistor (multiplier) to measure voltage. The galvanometer reads full scale at a specific current (e.g., 1 mA). To measure 0–10V:

Required series resistance = 10V / 0.001A = 10,000Ω (10 kΩ)

Make the resistor from resistance wire or carbon compound, calibrated to the correct value. Connect it in series with the galvanometer.

Multiple ranges: use different multiplier resistors with a selector switch.

Converting a Galvanometer to an Ammeter

Add a small parallel resistor (shunt) to carry most of the current. The galvanometer’s internal resistance must be known first—measure it with the Wheatstone bridge technique.

If internal resistance is 100Ω and full-scale current is 1 mA, to measure 0–1A:

• The shunt must carry 999 mA while galvanometer carries 1 mA
• Shunt resistance = (1 mA / 999 mA) × 100Ω = 0.1Ω

A short length of thick resistance wire, carefully cut and adjusted to give correct deflection at a known current, makes an adequate shunt.

Building an Ohmmeter

Connect battery + series resistor + galvanometer. Short the probes: the galvanometer deflects full scale (adjust the series resistor until it does—this is the zero-resistance calibration). Open the probes: no deflection (infinite resistance). An unknown resistor connected between the probes deflects the needle to an intermediate position proportional to its resistance.

Calibration marks:

Deflection (% of full scale)	Resistance (× mid-scale value)
0%	Infinite
25%	3×
50%	1× (mid-scale)
75%	0.33×
100%	0

Mark the mid-scale resistance value on the scale—it equals the series resistor plus galvanometer internal resistance.

Practical Use and Care

Protecting the galvanometer: Always use the highest current range first when measuring unknown circuits. A galvanometer is easily damaged by excessive current—the fine wire in the coil will burn. Approach the measurement from above.

Zeroing: Before each measurement, confirm the pointer rests at zero with no current flowing. Mechanical zero adjustment is usually a small screw that shifts the pointer. If the instrument was stored with a strong magnetic field nearby, it may need re-zeroing.

Vibration: The fine suspension springs in a sensitive galvanometer are vulnerable to mechanical shock. Mount instruments on vibration-dampened bases for accurate readings. Allow the pointer to settle before reading.

Mirror scale: High-quality galvanometers include a mirror strip on the scale. Read the scale at the point where the pointer and its reflection align—this eliminates parallax error from viewing at an angle.