Opinion pollers invented instant surveys in hopes of removing the burden of time from their work. When they place telephone calls to several thousand people and ask their views of candidates amid a hot campaign, they are trying to hit a moving target. Who knows whether Wednesday's mood has drifted away from Tuesday's? If so, what opinion are the pollers measuring? They also face pressure from clients demanding quick results: opinion as close as possible to real time. Technology has come to their aid. In 1973, the most powerful polling operation in the American economy, Nielsen Media Research, placed new boxes on its subjects' television sets, linked directly by telephone lines to the company's central computers. This system, dubbed "Storage Instantaneous Audimeter," let the company's operations center in Florida track the ebb and flow of viewer behavior minute by minute. That fine temporal grain was a beginning, not an end. Film studios now routinely test their product by letting audiences watch with a dial in hand, registering a sort of instantaneous electric approval. But approval of what?