Experiences

The solenoid. Indirect measurement of the earth's magnetic field

Gomboli Sandro

Italy

Vocational School

Physics

Laboratory

Direct

Knowing the geometric characteristics of a solenoid and the intensity of current flowing through it, it can indirectly measure the magnetic field induced inside.
Place the axis of the solenoid in east-west direction, a compass is placed inside.
The intensity of the current of the circuit is set up to bring the index of the compass at 45 ° to the axis of the solenoid. In this state, the magnetic field induced by the solenoid is equal to the terrestrial and therefore indirectly we can measure the value. With simple devices can be measured, even approximately, a physical invisible and impalpable as the Earth's magnetic field.
This experience involves the use of a solenoid of great dimensions, connected in a series, to an elementary electric circuit powered by low tension direct current.
An analogical ammeter enables to measure the intensity of the current circulating.
To measure indirectly the earth's magnetic field we apply the properties of electromagnetic induction.
The axis of the solenoid is east-west oriented and a compass is placed inside, in its geometric centre.
The compass, when the circuit is on, will point to the magnetic North perpendicularly to the axis of the solenoid.
When the circuit is off, the current passing through it, will create an electromagnetic field with a force tending to orientate the magnetic needle of the compass to the axis of the solenoid.
The magnetic field of the compass is subject to the Earth's magnetic field and the magnetic field induced by the solenoid, and when, varying the current, the two electromagnetic forces are equivalent, the needle places at 45° to the axis of the solenoid as for the second rule of parallelograms.
In this condition of balance, the magnetic field induced by the solenoid is the same as regards its modulus as the Earth's magnetic field.
Given that we know the geometric characteristics of the solenoid and the intensity of the current passing through it, we can measure indirectly the magnetic field induced and therefore, by analogy, the Earth's magnetic field with an absolute uncertainty, due mainly to random errors which are not marginal in this experience.
These random errors are mainly due to two factors:
1. The magnetic field measured is the one existing in the laboratory, which does not coincide exactly, with the Earth's magnetic field., but is also determined by the presence od electric circuits and electromagnetic appliances.
2. The direction of the magnetic needle of the compass is determined very approximately, owing to parallax errors, the instrument's sensitivity and the inaccuracy of its positioning.
These errors could be reduced, even remarkably, if better instruments and equipment were available.
This experience is meant to make the students aware of the importance of method in scientific enquiry: it is possible to measure, even if approximately, with simple instruments, an invisible and impalpable quantity such as the Earth's magnetic field.