Magnetic Fields of Currents

Author: J.B. Hoag

It has been established that a magnetic field is produced whenever an electrical charge moves. If the charge is at rest, only the electrostatic field exists. Thus a magnetic field exists around the electrons moving inside an atom, around a beam of electrons moving through a vacuum tube, and around the free electrons of a metal as they drift down the wire under an impressed potential.

Fig. 3E. The magnetic field around a wire carrying a current

Figure 3 E shows how, with small compasses, Oersted found that the magnetic field around a wire carrying a current exists as complete, concentric circles, in planes perpendicular to the wire.

Fig. 3 F. Principle of a galvanometer, ammeter, or voltmeter

When a current is sent through a coil of wire properly suspended in a magnetic field, as in Fig. 3 F, the combination of the two fields causes the coil to rotate, and by an amount proportional to the current. If a mirror is mounted on the loop, the rotation can be measured by the deflection of a spot of light. Such a current measuring instrument is called a galvanometer and proves to be quite sensitive to small currents, measuring down to one one-billionth of an ampere. If the loop of wire is pivoted in the magnetic field, has a spiral of wire to restrain its motion, and a pointer is fastened to it, a more rugged current meter is obtained. Then, with suitable resistance shunts around the coil, the instrument is called a d.c. ammeter and is calibrated to read amperes directly. Often in radio circuits, the shunt is so chosen that the meter reads directly in milliamperes (= ma. = one one-thousandth of an ampere = 10^-3 amp.) or in microamperes (= μ,a. = one one-millionth of an ampere = 10^-6 amp.) If a high resistance is connected in series with the loop, the meter may be connected across a circuit and used to read the voltage drop in that circuit. It is then called a voltmeter. If the coil is mounted so as to rotate freely, and a "commutator" is added, we have a motor.

Fig.3G Magnetic field of a solenoid

The magnetic field around a wire, coiled into the form of a solenoid, as shown in Fig. 3 G, is the same as that around a bar magnet, with a north pole at one end and a south pole at the other. This is called an electromagnet. Its strength can be varied by changing the amount of the current flowing through it. If an iron core is placed inside the solenoid, the magnetic field becomes much greater.