The number of working oil and gas rigs in the United States this week totaled 917, down five from the previous week, Baker Hughes reported. "It's just a random fluctuation; I wouldn't read that much into it," said Ike Kerridge, spokesman for Baker Hughes. "I think it will continue to rise (over time), but it could go up or down week to week." A year ago, the count totaled 716, the Houston-based oil toolmaker said. The rig count is considered to be the yardstick of drilling activity. Hughes Tool, which recently merged with Baker International of Orange, has kept track of the number of working rigs since 1940. A little more than a year ago, the rig count fell to 663--the lowest since Hughes began compiling figures. The count reached a peak in December, 1981, the height of the oil boom, when 4,500 rigs were active.