By the numbers

8

0.5

7

The 726 used what is now generally known as 7-track tape, which features six tracks plus one parity track for a total of seven parallel tracks that span the length of the tape.

6

Data is stored on the 7-track tape as 6-bit characters, with each character and the additional parity bit stored in a different track.

100

The IBM 726 had a recording density of 100 characters per inch.

12,500

The 726 tape reel was equivalent to 12,500 punched cards.

75

The 726 moved at a rate of 75 inches per second.

7500

The transfer rate for the 726 was 7500 characters per second.

1

There was a one-inch gap between records to allow the mechanism time to start and stop the tape.

10

The start time and stop time for the 726 was 10 milliseconds.

1400

The 726 had a 1400-foot maximum reel length.

In the late 1940s, magnetic tape was showing its usefulness in the audio recording industry, and Thomas J. Watson Jr., then executive vice president at IBM, felt it was time to explore magnetic tape as a data storage medium as well. But the tape was too fragile and prone to breakage if run at high speeds—it had to start and stop quickly as data was recorded and read. Before magnetic tape could become a viable storage option, a method for protecting the tape from breakage had to be invented.

IBM engineer James A. Weidenhammer was part of the IBM team looking for an answer. He tried using a vacuum cleaner to blow air onto the tape in the hopes that it would form the tape into loops and prevent it from binding and breaking. It didn’t work. But then, Weidenhammer recalls, “for some reason or other I just switched things around ... and it worked much better.” The vacuum pulled the tape down from the bottom of the column, keeping the tape from sticking and breaking during the rapid accelerations and decelerations. Its use in the IBM 701 signaled the beginning of the era of magnetic storage, for its buffering technique would become widely adopted throughout the industry.

Magnetic tape becomes a viable solution

“It was a unique solution that provided essentially uniform tensions across the tape and allowed the tape to be brought up to full speed in 7.5 ms on the first two systems, and around 1 ms in later ones. The gentle handling enabled the use of more fragile, but lighter, plastic-backed tape. This was an unbeatable combination for high-performance tape systems and was used from the 1950s through the 1970s.”

Daniel, Eric D., C. Denis Mee and Mark H. Clark

Magnetic Recording: The First 100 Years, page 258

A new world record in data density

In 2010, IBM demonstrated a new world record in areal data density on linear magnetic tape with a prototype that has a density of 29,500,000,000 bits per square inch—enough to hold 35 terabytes (TB) of data.