How to Measure the Density SG of Rock: A DIY Prospector Tool Rule

How to Measure the Density SG of Rock: A DIY Prospector Tool Rule

The determination of specific gravity dates from such antiquity, and the various published methods of determining it are so numerous, that one may well be skeptical as to the value of a new means of obtaining so well-known a quantity. I make no claim to any great novelty for the little device here described, the principles, though not the convenient details, of which are more or less embodied in the publications to which my attention was drawn after I had made the instrument.determination-of-specific-gravity-by-the-density-rule

My desire is to render a service to explorers and prospectors, who go to more or less inaccessible parts of the earth, and to whom any convenient means of determining the nature of the minerals they find is of the greatest possible value. Now the color, crystalline form, hardness and density go a long way towards such determinations, and, with the exception of the last, can be ascertained by observation and the use of a little box (that need not be larger than a pocket match-box), containing the specimens which represent the scale of hardness. The determination of density, however, has necessitated hitherto the use of a balance or other cumbersome apparatus; and my purpose is to replace this by a very light and convenient little instrument, no larger than a pencil, that may be carried in the pocket without inconveniencing the traveler.

A three-sided ivory rule, 21 centimeters long and 0.5 centimeter in width of side, is engraved on one side with a centimeter-scale, from 0 to 10 at the center, and again from 0 to 10 from the center to the other extremity, leaving half a centimeter over at each end. On another side is a scale of density, constructed so that the value at any point is always obtained by dividing the whole length from the center to the extremity by the distance from this point to the extremity. In the center of the rule is drilled a hole, through which a cord is passed; and the rule must balance when suspended by this cord, with the two engraved scales upwards.

A brass bob, 10 grammes in weight, is made so that it can


slip on the rule. It has two openings to permit the reading of either scale, and at the center of each of these openings is an index-mark, which must also be at the centre of weight of the bob.

To make a density-determination, select a specimen somewhat over 10 grammes in weight; place the brass bob with the index-marks at zero on the density- and millimeter-scales; suspend the specimen by a horsehair or silk thread from the other arm, at any point where it will exactly counterpoise the bob, and fix it there, which may conveniently be done by having a running loop on the horse-hair. Now immerse the specimen in water, and move the brass bob back until it again counterpoises the specimen. On the density-scale the index of the bob will show the density of the specimen, with accuracy to the first place of decimals for any ordinary specimen, from a density of from 2 to 5. From 5 up to 10 it will read to half a unit; and thereafter it is only approximate, but will show roughly, for instance, whether gold be ” good ” or “ bad ” in fineness.

The principle is obvious. If the bob, whether it be of 10 grammes or any other arbitrary weight, counterpoise the specimen when at some fixed point on the other arm, and, after moving inward, again counterpoise the specimen when immersed in water, obviously the distance that the bob is moved inward represents the loss of weight in water; and this distance bears the same ratio to the length from the center to the extremity of the scale as the weight of an equal bulk of water bears to the weight of the specimen. If the specimen tested weighs less than 10 grammes, sufficiently accurate results can be obtained by using any little pebble suspended from the other arm as a counterpoise in place of the brass bob.

Table I. gives the position of the points that should be engraved on the density-scale in centimeters from the end of the scale, and will be of use to anyone making an accurate scale with a dividing-engine.


Table II. gives a few densities taken by the rule, and also accurately determined by a more elaborate method, from which it will be seen that the former determinations are close enough to satisfy the requirements of a prospector, especially when it is considered that they can be made with ease at the rate of more than six in a minute.


Fig. 1 shows the position of the specimen and bob, at the moment of the final weighing in water, after the bob has been moved into its new position. In Fig. 2, the scale on one arm of the rule, drawn to actual size, is also given for the convenience of those who desire to make a rough scale of the kind, and have not the means at hand to get it accurately divided.