"The voltmeter is converted into the fuel gauge with a uniform scale." Your sentence is grammatically correct, but it's not true. A voltmeter doesn't convert anything nor is it converted by anything, rather it is an instrument used for measuring the potential difference, or voltage, between two points in an electrical or electronic circuit.

The voltmeter is converted into the fuel gauge with a uniform scale. Well, sort of. An engineering description, so not sure what "natural" means here. :) Not quite clear the relationship you want to express for the the voltmeter and the fuel gause. One is used as the other? Depends a little on context. The voltmeter becomes a fuel gauge with a linear scale. (I would ude "linear" over "uniform") The voltmeter acts as the fuel gauge with a linear scale. The voltmeter acts as a fuel gauge with a linear scale. The voltmeter forms a fuel gauge with a linear scale. P.s. I'm an electronics engineer.

Here this device is designed to use the voltmeter as a fuel gauge with a uniform scale, so the volume of the oil can be seen.

Thank you all very much! I am translating a physics testing paper into English for international students. Here is the context. I am sorry the context is my translation if it is not clear. What is shown below is a schematic diagram of an automatic oil volume measuring instrument. O is the leverage point; R0 is a fixed resistance; Rx is a sliding rheostat.When switch S is closed, which of the following statements is correct

No, it isn’t natural or even clear. You need more explanation and “converted” is probably not the best word, though maybe it would be ok depending on the context. It sounds like you might be trying to say “the voltage (somewhere) causes the fuel gauge needle to deflect and (something happens). Also “uniform” is unclear.