Electromagnet for Sonometer

Quick Answer: The Electromagnet for Sonometer is a coil wound on an iron core that is placed close to the steel wire of a sonometer and fed with low-voltage AC. Its alternating magnetic pull sets the wire vibrating, so students can tune the wire to resonance and determine the frequency of the AC mains.

Driving a Sonometer Wire Without Touching It

On a sonometer, a tuning fork normally sets the wire vibrating. The electromagnet for sonometer does the same job electrically. When AC flows through its coil, the core becomes a magnet whose strength rises and falls with the current, attracting the steel wire during each half-cycle. The wire is pulled twice for every cycle of the supply, so it is driven at twice the mains frequency.

Students then move the bridges until the length of wire between them resonates, a condition shown clearly by a small paper rider placed at the middle of the length jumping off. From the vibrating length and the tension, the frequency of the wire follows from f = (1/2l) x square root of (T/m), where m is the mass per unit length. Halving it gives the mains frequency, which in India should come out close to 50 Hz.

Because the drive is magnetic, the wire must be steel or another ferromagnetic material. A brass or copper wire will not respond; for those, the alternative method passes AC through the wire itself with a Horseshoe Magnet across it, in which case the wire vibrates at the mains frequency rather than twice it.

Applications

• Determining the frequency of AC mains with a sonometer
• Demonstrating resonance and stationary waves on a stretched wire
• Checking the relation between frequency, tension and vibrating length
• Comparing electromagnetic and permanent-magnet methods of driving a wire

Specifications

Care & Handling

• Power the electromagnet for sonometer only from the low-voltage transformer used for the experiment, never directly from a wall socket.
• Switch off between readings; the coil warms during continuous use.
• Place the core close to the wire without touching it, so it drives the vibration without damping it.
• Keep leads tidy and away from the vibrating wire.

Why Choose LabEquip

Physics departments running the mains-frequency practical usually buy the electromagnet together with the Step Down Transformer for the sonometer experiment. LabEquip lists this accessory in its General Items. Send the number of sonometer stations through the contact page.

Frequently Asked Questions

Why does the wire vibrate at twice the mains frequency?

An unmagnetised steel wire is attracted to the electromagnet whichever way the current flows. The pull reaches a maximum twice in every AC cycle, once for each half-cycle, so the wire receives two pulls per cycle and vibrates at double the supply frequency.

Does the sonometer wire have to be steel?

Yes, for this method. The electromagnet can only attract ferromagnetic material. Brass, copper or nichrome wires will not be driven; they need the method in which AC flows through the wire between the poles of a permanent magnet.

How do I know the wire is at resonance?

Place a small folded paper rider at the middle of the length between the bridges and slowly change the length. At resonance the wire vibrates with maximum amplitude and the rider is thrown off.

How is the mains frequency calculated?

Find the resonant length, then calculate the wire frequency from the tension and mass per unit length using f = (1/2l) x square root of (T/m). Divide that result by two to obtain the mains frequency.

What power supply does it need?

Low-voltage AC from a step-down transformer, as used in the standard practical. It must never be connected directly to the mains, and its rated coil voltage should match the transformer output.

Why does the electromagnet get warm?

Current flowing through the coil heats it, and the alternating field also produces some heating in the iron core. Switch off between readings and do not leave the circuit on unattended.

Last Updated: September 2026