Line-powered, more expensive, but we think it is worth it. There is no AC adapter to lose or batteries to replace just before the lab starts. Our six-digit electronic stop clock provides memory and period measurement with a resolution of 0.0001 s. The timer base is generated by a 10-k Hz quartz crystal with a lifetime accuracy of ±0.02%. The clock can be operated with one or two photogates, measuring the interruption of a single photogate or the time between breaking a pair of gates. Includes one photogate.