Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Oscilloscope and alternating voltage from transformer


A way of showing quantitatively the relationship between the e.m.f. and turns ratios of transformer coils.

Apparatus and materials


Demountable (demonstration) transformer with coils of 12 000 and 300 turns

Leads, 4 mm, 2

Health & Safety and Technical notes

The 12,000 coil must be designed for use on the mains, i.e. the mains lead must be permanently attached or connected via an IEC connector.

The mains lead to the 12,000 turn coil should be permanently connected; 4 mm leads connected to a 13 A mains plug are a serious hazard. 

If the coil fitted with a mains connector has only 1,200 turns, reduce the secondary to 30 turns. Do not improvise mains connections with 4 mm leads. 
A video available at the National STEM Centre eLibrary shows how to use an oscilloscope.



Apparatus set-up Photograph courtesy of Mike Vetterlein

a Set up the demountable transformer with the 12,000-turn coil on one side of the U-core and the 300-turn coil on the other. 
b Plug the 12,000-turn coil into the (230 V) mains. There will then be about 6 volts across the secondary coil when the core is completed. 
c Connect the low-voltage output from the secondary of the demountable transformer to the Y-plates of the oscilloscope by direct connection to the input terminals. 

Teaching notes

1 The frequency of the AC mains can be calculated from the time-base reading. 

2 The turns ratio and the voltage ratios can be compared. 
This experiment was safety-checked in April 2006


Related guidance

Explaining how a transformer works


Cookie Settings