Early Morse equipment. At that time (mid-19th century), it wasn't possible to make an oscillator as amplifying valves didn't exist, so the pen recorder was the only option available. It's also the source of the nickname still used (particularly in the US) for a poor operator. To help them decode it by listening to the clicking solenoid, trainee telegraphists used to stick the lid of a tobacco tin, or similar, on top of the cabinet holding the solenoid; this acted a bit like a speaker cone, resonating as the solenoid clicked in and out - so a novice or poor operator was termed a 'lid'!

In Britain, early railway systems linking signal boxes used a solenoid which was biased to a centre position, and could be swung either left or right to hit one of two small gongs, which rang with two different notes. The 'dots' and 'dashes' in that system could be more accurately termed 'marks' and 'spaces', as both were the same length - basically, frequency shift keying! The effect was as though CQ was transmitted as:

tong ting tong ting . . . tong tong ting tong