Conversation so far:
When did the inventor Lee de Forest work on Arc radiotelephone development ?
On July 18, 1907, de Forest made the first ship-to-shore transmissions by radiotelephone.
How did the inventor Lee de Forest do Arc radiotelephone development?
Using his sparkless arc transmitter, de Forest first transmitted audio across a lab room.

Question: What did he transmit in that experiment ?
Self-contained rewrite: What did Lee de Forest transmit in his 1906 experiment ?