Abstract: Reversible, elapse-time indicator utilizing two solid electrolyte cells in which current of one polarity is applied to one cell to cause silver ions to move from the positive electrode to the negative electrode. When all the ions have been removed from the positive electrode after a predetermined specific interval of time, the cell resistance increases to trigger an alarm such as a lamp. While the first cell is timing out, the second cell is being charged from a zero time value by a current of an opposite polarity equal to the time begun within the first cell. When the time interval has elapsed, the cells are reversed so that the second cell now becomes the timing device and the first cell is being recharged.