What caused the shift from calling things like rheostats and condensers to resistors and capacitors, or the move from cycles to Hertz?
It seemed to just pop up out of nowhere, seeing as the previous terms seemed fine, and are in use for some things today (like rheostat brakes, or condenser microphones).
"Resistor" usually implies a device with a fixed resistance value. A rheostat is a device with variable resistance. The two terms are not synonymous.
As for condenser and capacitor, Wikipedia has an interesting tidbit:
Early capacitors were known as condensers, a term that is still occasionally used today, particularly in high power applications, such as automotive systems. The term was first used for this purpose by Alessandro Volta in 1782, with reference to the device's ability to store a higher density of electric charge than was possible with an isolated conductor. The term became deprecated because of the ambiguous meaning of steam condenser, with capacitor becoming the recommended term in the UK from 1926, while the change occurred considerably later in the United States.
So, that’s where Finnish borrowed that word… like so many other words too. Perhaps calling it borrowing isn’t entirely fair, since this thing has been going on for so long and it’s been really extensive. Sort of like the way the British Museum “borrowed” a significant part of their collection from somewhere else.