The Secret Signal: Tesla's 1903 Patent for Private Wireless Messages (U.S. 725,605)

2 minute read. Published October 17, 2026.

In short: U.S. Patent No. 725,605, System of Signaling, was granted to Nikola Tesla in 1903. It describes sending a wireless message on several frequencies at once, with a receiver that only responds when all of its tuned circuits are triggered together. A stray signal on a single frequency does nothing, which makes the system more private and less prone to interference.

Tesla's 1903 System of Signaling patent drawing printed in amber on a black T-shirt — The drawing from U.S. Patent 725,605, restored and printed in amber on black.

What problem did it solve?

In the first years of radio, a message sent on one frequency could be picked up by anyone tuned to it, and nearby transmitters could easily interfere with each other. That was a serious problem for anyone who wanted wireless to be reliable, or confidential.

How did it work?

Instead of relying on one frequency, Tesla's transmitter sent its signal using a combination. The receiver contained several circuits, each tuned to one part of that combination, and it was arranged so that it only acted when every circuit responded at the same time. Think of it as a lock that needs several keys turned together.

Why does it matter?

It's an early answer to a question communications engineers still work on: how do you make sure only the intended receiver hears the message? Ideas about selectivity and combining signals run through much of the radio technology that followed.

What does the drawing show?

The drawing lays out the tuned circuits and their connections in Tesla's characteristic technical style, one of the more intricate diagrams in the collection.