Two small electromagnets are used which act upon a small soft iron armature connected to a pointer. The armature will position itself between the cores of the electromagnets depending on the magnetic strength of each. The ratio of magnetism in each core is changed as the linear variable resistance sender changes and hence the needle is moved. This type of gauge reacts very quickly (it has a small time constant) and is prone to swing about with movement of the vehicle. Some form of external damping can be used to improve this problem. Resistor R1 is used to balance out the resistance of the tank sender. A good way to visualize the operation of the circuit is to note that when the tank is half full, the resistance of the sender will be the same as the resistance of R1. This makes the circuitbalanced and the gauge will read half full. The sender resistance is at a maximum when the tank is full.
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