Try to get someone who is good at speaking in public to give a 2 to 5 minute talk about the history of the instrument. They should use short sentences to make simple statements. The idea is, the TV station can look at that and realize they can clip out a number of prominent statements (like "The Theremin was patented in 1921 by Prof. Lev Termen") and assemble them to fill a desirable amount of time. Stations decide in advance how much time to give a story, and then decide how to break up that time between their reporter talking, you talking, and "action", so if for example they want 30 seconds of you talking and you give them a bunch of 10 second sentences, they'll use three of them. (On the other hand if you give them a bunch of 45 second sentences, they'll use none of them.) If you make it easy for them to do this, the station will like you and you'll probably get more information across.
Also, if you make the information simple and easily understood, their reporter will probably quote some of the information you said, even if they don't show you saying it.
This little talk should also be memorized, so you don't look like you're reading when they point a camera at you.
Also make a point to take the reporter and camera-person aside in advance and ask them how you can help them best get their story. (They may want, for example, to adjust your seating arrangement to show you more easily on camera, or they may want to suggest that you do or do not look at the camera, or to interview you before or after the meeting.) By helping them, you give them incentive to portray you positively. And so they understand your perspective, tell them honestly what you want to get out of it - exposure for the instrument as an actual instrument, not as a novelty sound effect. It's okay for you to have goals, as long as you're honest about it. Your goals don't have to conflict with theirs - as long as they can see you have nothing to hide, they can report it honestly and it can also serve your purpose.
