The television commercial is not that seemingly innocent 30-or 60-second message from the sponsor but an instrument of power wielded in various and mysterious ways undreamed of by the consuming public. The exercisers of this power, the potentates of the title, are global corporations that want to control supply at one end and demand at the other. To do this, they endeavor to sell the unnecessary--the necessities will be bought anyway. So the focus is on the creation of ""emotion-charged values to make the unneeded necessary."" Their message: salvation through consumption. Barnouw, perhaps broadcasting's premier historian, develops this theme by tracing the evolution of that ""almost mythological figure,"" the sponsor, from the first one, AT&T, through the days of radio when he sat in splendid arrogance in the fabled sponsor's booth. Today, he sits by the ratings computer, scarcely looking at programs but concentrating instead on charts and Nielsen numbers. But his influence, demonstrated through his ingeniously crafted commercials and pocketbook, has not waned. One melancholy statistic: the search for topics acceptable to sponsor-money has almost ended low-rated network documentaries. More disturbing: producers and writers are so attuned to sponsor desires that they automatically avoid controversial areas. Thus, the ""tamed artist."" A small, well-reasoned, and scholarly book that will trouble the thoughtful viewer and infuriate Madison Avenue.