The purpose of the device wasn't made clear, although it could have been a type of laser gun, although it didn't look much like a gun, and it is also worth mentioning that he already had a ray gun, and it's not likely he would build another. Another possibility is it may have been some type of lamp or another illumination device. This also seems unlikely as it doesn't seem to fit with his desire to further the science of his miniaturizing ray.
One other possibility was that it could have been used as another shrinking machine. Whereas his previous machine was only able to shrink people that were listening to the radio or watching television; a machine that could send his microwave miniaturizing ray through light itself would be advantageous. If he was to build a larger satellite model, he could have put it into orbit and projected his microwave shrink beams in the form of a light ray, that would have covered the Earth in his shrink ray.
But all of this is merely speculation. Whatever the device was meant to be used for, it was apparently designed to be tested in the dark, with all of the lights out.