Spirit-levels are often made of wood, which is very objectionable for the erector’s use, because the lower or testing surface is apt to catch and hold particles of metal, and furthermore it is very susceptible to abrasion, and wears rapidly. It is preferable, therefore, that it be of iron or steel. The test of a spirit-level is its sensitiveness, and it is found in a properly constructed one that the bubble will move to a perceptible extent if a piece of gold leaf be inserted under one end. In a spirit-level which came into the hands of the author of this work he found the warmth of the finger when placed on its top sufficient to cause the bubble to move nearly the full length of its tube, the body of the level being a block of iron 11⁄4 inches square and 9 inches long. The movement of the bulb was caused by the heat of the finger expanding the top of the spirit-level and causing it to bend. To test the truth of a spirit-level, it should be placed upon a true surface, as a surface plate, and if the bubble comes to rest at the same spot in the length of the spirit tube when the level is tried turned end for end, the level is true. The test should be made several times.
Fig. 2450.
The plumb-rule, though less used by machinists than formerly, is better for machinists’ use than the ordinary wooden-bodied spirit-level, since it is more delicate if properly constructed. It should be formed as in [Fig. 2450], the sides a a and b b being straight and parallel one to the other; c and d are two plugs of soft yellow brass let in so as to keep the line l l clear of the face of the level, so that there shall be no friction between them. At n are notches to secure the line, which should be as fine and as closely spun as possible.
Fig. 2451.
The plumb-level, [Fig. 2451], is also preferable to the ordinary spirit-level; its edges a, b must be straight and at a right angle one to the other, c and d representing brass plugs as before. The edge a of the rule or of the level should be laid upon a surface plate, and a fine line drawn on the face of these plugs with a scribing block, the coincidence of the line l with these marked lines testing the truth of the work.
Fitting or Making Joints.—The best form of joint to withstand pressure is the ground joint, and next to this, but more expensive, is the scraped joint. The difference between the two is as follows:—