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AR15.COM
10/19/2015 8:43:09 PM EDT
I just received a czech morse key and the seller threw in a vacuum relay. Not to sound ignorant, but what do you do with a vacuum relay with morse code?
10/19/2015 8:53:47 PM EDT
[#1]
I'm guessing to key the transmitter in an area where combustible fumes may be present like in an aircraft. Key operates relay which is located at the transmitter unit in the bowels of the plane.
This is just a guess though.
10/19/2015 8:56:39 PM EDT
[#2]
It's used for switching an antenna between a receiver and a transmitter, mostly at high power levels. Definitely old school.

Today, it's mostly for high voltage
10/20/2015 3:09:07 PM EDT
[#3]
Please post a picture. He might have used it as an interposing relay between the key and his older radio. Some old radios have high switching voltage and an interpose relay is used for safety.
Many high power RF amplifiers use vacuum relays to switch its output TX / RX.  The reason is that they don't arc and make very little noise when switched. The low noise is important for full QSK CW operation.
There are many more applications for vacuum relays. I used small ones for switching high voltage loads in my home brew projects. I have used  vacuum capacitors  in antenna tuners and amplifiers as well.
10/20/2015 3:47:07 PM EDT
[#4]
In theory vacuum relays are "faster" than mechanical relays, among other benefits already mentioned.