One embodiment of the present invention provides a system that facilitates measuring quality-of-service of a computer network server or collection of servers by determining the effect that transactions in one category of service have on transactions in other categories of service. One example of such hollow balls is the game ball used in the pickle ball game. A rate of change in the rotational speed of the electric motor with respect to the operation of the operating unit in the transition region is greater than those in the starting region and common-use region. The first circuit may sequentially step through a plurality of different output states in response to a clock signal.