To check if a transformer in a neon sign is functioning, which instrument can be connected across the secondary terminals?

The most appropriate instrument to check if a transformer in a neon sign is functioning by connecting it across the secondary terminals is the milliameter. This is because a milliameter is specifically designed to measure small currents, which is ideal for assessing the output of a neon sign transformer.

When evaluating the output voltage from the secondary terminals, a milliameter can provide a clear indication of whether or not the transformer is delivering the expected current to the neon sign. Additionally, transformers used in neon signs typically operate at high voltages and low currents in the secondary, where a milliameter's sensitivity can effectively measure this output.

Using a standard voltmeter is not typically effective in this scenario, as transformers can output high voltages that may not be within the safe or effective range of standard voltmeters. An ohmmeter, on the other hand, measures resistance rather than current or voltage, which is not relevant in this case. Employing a jumper is not an appropriate method for checking functionality, as it could lead to short circuits or damage. Thus, the milliameter is the best choice to accurately assess transformer operation in this context.