This is a general diagram, it is similar to the oil pressure and temperature units. They use a bi-metal arms that is heated by the current passing through a coil. The current is controlled by contact points, so it is a type of pulse system. Variation in the condition (in this case fuel level) is measured by changing the pressure on the bi-metal arm what changes the current required to make / brake the contact points.
Added some better diagrams from a post by 19Fordy on the Barn. These diagrams are for a 51 Merc.
Do not believe you can test these circuits like you are thinking. It is a current device, not a voltage device. The last attachment has a test to isolate the problem between the sender and the gauge.
Also might read post
How to calibrate oil pressure gauge? same issues, same type of gauge.