Simple Experiments to show
the magnetic field due to a current
in a solenoid.

See Single Turn  NOTE SEE New experiment

I constructed a wooden frame 25 cm by 25 cm to hold a coil of four turns 25 cm in diameter 25 cm long of 16 gauge (1.6mm) tinned copper wire.  A resistor of 15 ohms and a switch were placed in series.  A regulated power supply unit supplied 20 volts giving a current of 1.33 amps.

The diameter at the mid point of the coil was lined up with the horizontal component of the Earth's field at that point.   Measurements were taken at 5 cm intervals across the diameter with the centre as one of the points.  At each point the deflection of the magnetic compass due to the current was measured as an angle from magnetic north. 


The angle the compass takes up is along the direction of the vector sum of the Earths Field and the field due to the current in the loop.

Tan (Theta)=M/E
So M=E Tan (Theta)
Text Box: Tan (Theta)=M/E
So M=E Tan (Theta)
M field due to current
H Field due to Earth
Angle Theta

Photograph of Apparatus:


Distance from the centre (cm) Deflection (degrees)
-10 45
-5 45
0 45
5 45
10 44


This shows that the field near the mid point of a solenoid is constant across its diameter.  This also means that a coil carrying a current cannot be used for containment.

This result is entirely consistent with the Gauss construction of magnetic shells.


I did this experiment because I wanted to test a calculation I had done using the Biot-Savart hypothesis to find the off-axis magnetic field of a solenoid.  My predicted result was not supported by my experiment thus refuting my mathematics and all my conclusions.  I should have been shown this at school.  If I had been shown a demonstration instead of being subjected to brain washing then my life would have been better spent.

It also shows that, because there is no potential minimum, a coil cannot be used to contain a plasma.

I think that this demonstration should be carried out at school as part of the elementary general physics course

Further experiments using a lower current did support my calculations see below.