Is it permissible on shabbos to record the passage of time as follows:
A rotating arm (of a type of clock) revolves at a known rate. A stack of cards can be pushed against the rotating hand so that the hand picks up a card from the first pile and deposits it on a second pile while it is travelling. The total amount of time that has elapsed while the stack of cards was pushed against the rotating hand can be determined by the number of cards that has been moved to the second pile.
Do you say that seeing as in this case the second hand of the clock is measuring time anyway (except you have contrived a convenient way to record that time) that there is no issur medidah or do you say that you have contrived effectively a new device which you activate (by pushing the stack of cards against the second hand) to create a new way of measuring time?
Answer:
The prohibition against measuring something on Shabbos is based on the idea that it is similar to commerce: The Sages prohibited measuring because this is something related to sales and transactions (see Shabbos 74; 149; Tosafos Shabbos 126).
This being the case, there is no prohibition on measuring “time”. The prohibition refers specifically to concrete measurements, and not to the abstract notion of time (for the same reason, Rav Moshe Feinstein (Orach Chaim 1:128) writes that the formal prohibition doesn’t apply to measuring fever).
Therefore, I don’t see any problem with the interesting device you suggest.
Best wishes.
