The ad appearing Wednesday in 14 newspapers around the country seemed ominous enough:

"A Strike In The Bell System Could Begin This Saturday Night," it announced in stark black print.

The ad by the Communication Workers of America spoke gravely about the union's desire to avoid "such a disruptive strike" and about its responsibility to inform the public" that danger was at hand.

But what the ad didn't say is closer to the truth. A walkout by the 525,000 CWA members and the other 176,000 unionized workers in the Bell System at midnight this Saturday might be inconvenient for the company, but it would do little to disrupt customer service.

The reason is automation -- specifically the burgeoning use of computers and miniscute "integrated circuits" to do work that once required legions of telephone operators and skilled craftspeople.

Since it began "cutting over" to dial telephones in the 1920s, the American Telephone and Telegraph Co., parent firm of the Bell System, has become one of the most automated organizations in the world -- one capable of running itself, delivering basic telephone services, for months without its full complement of employes.

Some examples are in order:

Last year, the Bell System handled 190 billion separate voice, data and graphics messages, many of them solely handeled by computers, nearly all of them computer assisted.

The company serviced 138 million telephones -- 5 million of them new installations -- in 1979. New technological advancements and Federal Communication Commission ruling made the terminal ware -- phones and other telephonic equipment -- easier to acquire and install.

The basic index of productivity in the telephone industry -- the number of employes per 10,000 phones -- fell from 148 workers per 10,000 units in 1950 to 60 workers per 10,000 in 1979.

Because of the increasing technological breakthroughs (an "acronym revolution," said one company official referring to terms like ESS, electronic switching system, and ACTS, automated coil toll service) the company needs more knowledge" workers and fewer "tool" workers.

That means the number of managers in the Bell System, which employed 1.042 million people in 1979, is expected to grow at a rate three times greater than that at which the number of its non-mangement-level employes. Its two largest groups of employes are in office and clerical, 354,028, and craftspeople, 225,204.

None of this, despite Wednesday's ads, has been lost on the CWA leaders.

In fact, the union lists job security -- translate protecting jobs from automation -- as the second major issue in the bargaining that began June 4. The first issue is wages.

The last time CWA struck nationwide was in 1971. That same year, CWA workers at the New York Telephone Co. stagged walkout that lasted from July 21 to Feb. 16. Both stoppages caused some inconvenience, particularly in the completion of construction projects. But neither caused widespread, significant disruptions in basic telephone services.

CWA President Glenn E. Watts, speaking on a radio interview show two days before his union's ad appeared, said: "I don't really like to predict one . . ." But should a strike occur, Bell System customers can expect the following, according to company officials;

No disruption of basic telephone service.

No delays in direct long distance calling -- long distance call unassisted by operators.

Delays in the fulfillment of directory assistance requests.

Possible, but probably brief delays in telephone installation.

"Priority attention" for emergency repairs, especially those affecting vital agencies -- health and law enforcement -- and businesses.

Thirty local CWA contracts, including one covering 31,750 employes of the Chesapeake and Potomac Telephone Co., also expire with the national pact at midnight Saturday.

As in the past, the local negotiations could yield several strikes, particularly in New York City, where CWA members installing communication's equipment for Monday's opening of the Democratic National Convention reportedly began a "slowdown" yesterday.

However, according to the Associated Press, AT&T officials in New York expressed confidence yesterday that they could complete all of the convention telephone hookups before the Saturday strike deadline.