A coil of current-carrying wire is otherwise known as a solenoid; it is often referred to as an electromagnet. The right-hand curl rule for solenoids states that your fingers curve in the direction in which the current flows through the coils while your thumb points in direction of the coil's magnetic moment , or the "N pole" of an electromagnet.

     

 

The right diagram shows a cross-section of the same coil. The black circles  represent where the current is flowing out of the coil and the black  represent where it flows into the coils. Using the right hand curl rule results in your fingers curling up towards you at the top of diagram and down in towards your wrist at the bottom. Consequently your thumb, the North Pole, or magnetic moment of this solenoid, must point down and to the right. Remember that magnetic field lines emerge from the North Pole and circle back into the South Pole.
 

We can use the following physlet to examine more closely the magnetic field inside of a solenoid. In the screen captures shown below, the "red" dots represent currents coming out of the plane of the page (+z) and the "blue" dots represent currents going into the plane of the page (-z). Thus, the right hand curl rule tells us that the magnetic moment, or North Pole, of this solenoid is located on its right side. Note that the magnetic field down the length of the solenoid is uniform and almost identical to that of a bar magnet.

 

 

 

The formula used to calculate the magnetic field within a solenoid is

 

B_solenoid = µ_onI

where

 

 

 

 

A device which uses the interaction between the magnetic moment of a current-carrying coil and the field of a permanent horseshoe magnet to detect the presence of current through the coil is called a galvanometer. This very sensitive meter movement can then be incorporated into other detection devices to quantitatively determine either the current through or voltage across various sections of an electrical circuit. Shown on the left is a picture of the demonstration galvanometer I inherited when I began teaching at Mainland in 1975. Although the springs are broken, you can still see the permanent horseshoe magnet, the current coil, and the needle which, when deflected, gauged the amount of current flowing through the coil.

 

To adapt a standard galvanometer movement into use as an ammeter you must place a low resistance shunt in parallel with the galvanometer’s meter movement.

 

 

Since the shunt is in parallel with the galvanometer’s meter movement, it will have the same voltage as the galvanometer. In addition, since it is in parallel, all the excess current will flow through the shunt to protect the galvanometer's meter movement. When you solve for the shunt's resistance r using Ohm's Law,

 

 

its value will be VERY SMALL.

 

 

 

To adapt a standard galvanometer movement into use as a voltmeter, a high resistance multiplier is placed in series with the galvanometer's meter movement.

 

 

Since the multiplier is in series with the galvanometer movement, it will draw the same current as the galvanometer. Being in series allows the multiplier to bleed off the excess voltage to protect the galvanometer. Once again, Ohm's Law would be used to solve for the resistance of this high resistance multiplier,