In the first controlled memory experiment, what memory measure did Hermann Ebbinghaus use to measure his own forgetting?

Trials to relearn.

Explanation:

In his first controlled memory experiment Ebbinghaus use trials to relearn the "nonsense syllables" (also known as the CVC trigram), which are a combination of a consonant, a vowel, and a consonant, that sounds like words, but they are not.

For the experiment, Ebbinghaus took several "nonsense syllables" and wrote them down. After that, in his second trial, he would write them in a period, then, at the end of the procedure, he would try to remember the syllables.