When a body resists forces that tend to pull it apart, the substance of which it is composed is said to be tenacious. Tenacity is a direct result of cohesion, a tenacious substance being one that has great cohesion. The tenacity of a substance is measured by the breaking weight per unit area of cross section.
The tenacity, or tensile strength, of steel is of great importance on account of the extensive use of this material in building operations. There are many grades of steel, but most of them have a tensile strength between 60,000 and 100,000 pounds per square inch.
Demonstration. - To determine the breaking weight of No. 24 copper wire, make a loop in one end and suspend it from a hook overhead. Fasten a pail to the other end so that it will be three or four inches above a table. Place known weights in the pail until the wire begins to stretch, then slowly pour in sand until the wire breaks.
FIG.1
By measuring the diameter of the wire and taking the weight of the total load, the tensile strength can be computed, by dividing the breaking weight by the area of cross section. (For example, if a wire 
in. in diameter is broken by a load of 104 lb., the tensile strength is 
lb. per square inch.)
In bodies of the same material, tenacity varies with the form of the body. When the areas of cross section are equal, a tube has greater tenacity than a solid cylinder of the same material, and a wire with circular cross section has greater tenacity than one with a square cross section.
Tenacity diminishes with the length of time the load is carried, so that a wire may finally break with a load that it would carry safely at first. Tenacity also diminishes as the temperature increases, on account of the increased rate of molecular vibration.
