Figure 1-2.Compass and conductor with direct current.
If the battery is disconnected, the north end of the compass needle will point to magnetic north, as
illustrated in figure 1-2(A) by the broken-line compass needle pointing to the right. When the battery is
connected, current flows through the circuit and the compass needle aligns itself with the magnetic field
of the conductor, as indicated by the solid compass needle. The strength of the magnetic field created
around the conductor is dependent upon the amount of current.
In figure 1-2(A), the resistance in the circuit is 6 ohms. With the 6-volt battery shown, current in the
circuit is 1 ampere. In figure 1-2(B), the resistance has been changed to 12 ohms. With the 6-volt battery
shown, current in the circuit is 1/2 or .5 ampere. The magnetic field around the conductor in figure 1-2(B)
is weaker than the magnetic field around the conductor in figure 1-2(A). The compass needle in figure 1-
2(B) does not move as far from magnetic north.
If the direction of the current is reversed, the compass needle will move in the opposite direction
because the polarity of the magnetic field has reversed.
In figure 1-2(C), the battery connections are reversed and the compass needle now moves in the
You can construct a crude meter to measure current by using a compass and a piece of paper. By
using resistors of known values, and marking the paper to indicate a numerical value, as in figure 1-3, you
have a device that measures current.