Okay, that too. But in order to measure the change in current through the resonant circuit, or voltage change created by a changing current flow, you need current to flow through the circuit under test. The flow was created because the circuit was radiating RF.
I built a few Heathkit devices, including a color television, an oscilloscope, and a printer. Heathkit did an extraordinary job of design in putting together their kits. The assembly documentation was second to none. The color television had at least 5 or 6 manuals, covering the assembly of the circuit boards, the mechanical assembly of the unit, tuning the varius stages, and adjustments to the set. It was a marvelous way of gaining experience in assembly and testing of complex devices. I don't think there are any equivalent systems today that even come close to the experience Heathkit provided.
As a power RF specialist I depedn on my trusty grid dipper. Mine happens to be an Eico 710. It's basically similar to the Heathkit shown but a bit more compact. I carry it in a canvas case originally made for a "brick" cell phone. The instrument and its coil set (The coils live in a mahogany block drilled to accept the pins.) fit perfectly. There is even room for a spare tube, a BNC coil adapter, and the Manual which fits in its own zippered compartment. I previously owned a solid state immitation "grid" dip meter, but it was almost useless so I traded it.
In addition to analyzing resonant circuits, the GDO can test transmission lines for electrical length and measure the distance to shorts or opens. As a wavemeter it detects RF leakage. Amazing versatility for an instrument with just one triode section. I won't leave home without it!
If you are curious about how you'd use a Dip Meter, check out my video on the basics of Dip Meters. In the video, I use a Heathkit Dip Meter, but it is a more modern, solid state version that I built as a kid in the 70s.