The TI-30 sold for less than the cost of a professional-grade slide rule. This calculating aid was a predecessor of the slide rule. The other side is a circular version of a slide rule. He is credited with inventing the slide rule in about 1622. Advertisements of the late 1930s compared the utility of the AVOmeter to the slide rule. The invention of the slide rule involved Oughtred in a priority dispute with Delamain. William Oughtred greatly improved this in 1630 with his circular slide rule. One slide rule remaining in daily use around the world is the E6B. Some airspeed indicators incorporate a slide rule mechanism to perform this calculation. Multiple speed-time-distance calculations can be performed hands-free at a glance with a slide rule. Nevertheless, Pickett, probably America's most successful slide rule company, made all printed scales. Throughout his life he never used any other slide rule. The device, called "meteoballistic summator", consisted of a specialized slide rule and a pre-calculated table. A slide rule has logarithmic scales, and nomograms often employ logarithmic scales. The E6-B is a circular slide rule used by pilots and navigators.