Re: Ammeter

Posted by HH56 On 2024/5/20 14:11:27

An ammeter is basically a loop of wire between the terminals and the size of the wire depends on how much current the ammeter is designed to handle since except for the starter motor almost all other current a car uses flows thru the ammeter. The loop can be actual wire or just a stamped piece of flat brass or copper bent in such a fashion as to form a loop. Using an ohmmeter to test between terminals should read a direct short but there should be no reading between either terminal and the case. The purpose of the cardboard is to keep the bulky wire terminals from touching the case if the wires got pulled or terminals are accidentally bent.

A simple explanation on operation is that as current passes thru the loop of wire it turns it into a weak electromagnet with north and south poles. That magnetism attracts a piece of iron that is also slightly magnetized and attached to the needle. When no current is flowing the needle is kept in the middle, sometimes by gravity of the piece of iron attached to the needle and in other designs by a small hairspring. In a simple car ammeter, if the car is charging the more the current is flowing from the generator thru the ammeter to the battery the stronger the magnetic field in the loop of wire will be and the more the needle moves. If the current flows in the opposite direction such as when a discharge is occurring from the battery the same thing happens only the magnetic poles in the loop of wire are reversed so the needle moves the opposite direction.