Check out these Voltage Indicator images:
Image by 3D King
3 x 12volt 7.5 amp batteries in parallel provide some outdoor power away from the grid
ARU-2B/A Power Up
Image by KLaFaille
A real ARU-2B/A Attidude Indicator being supplied power on my bench. This instrument is used in the A-10 and C-130 as well as many other US military aircraft.
28VDC is going into the Static Converter which converts and outputs the voltage to 115V and 26V 400Hz AC. The 115V AC is fed into the rear of the ADI which then routes the power through a transformer pack located on the rear of the instrument to supply the internals with the necessary voltages. The loud hum is the static converter and transformer pack.
The ball is spinning because there are no command signals being fed to it either by synchros (control transformers) or a digital to synchro controller.