“c. The beam must be sufficiently strong and inflexible to bear without bending the greatest weight that the construction of the balance admits of.
“d. The arms of the balance must be of equal length; i.e. the points of suspension must be equidistant from the fulcrum or point of support.
“The SENSIBILITY or DELICACY of a balance depends upon the following conditions:
“a. The friction of the edges upon their supports must be as slight as possible.
“b. The centre of gravity must be as near as possible to the fulcrum.
“c. The beam must be as light as possible.”
The following are the tests given by the same authority for the accuracy and sensibility of a balance:
“1. The balance is in the first place accurately adjusted, if necessary, either by the regulating screws, or by means of tinfoil, and a milligramme weight is then placed in one of the scales. A good and practically useful balance must turn distinctly with this weight; a delicate chemical balance should indicate the one tenth of a milligramme with perfect distinctness.
“2. Both scales are loaded with the maximum weight the construction of the balance will admit of; the balance is then accurately adjusted, and a milligramme added to the weight in one scale. This ought to cause the balance to turn to the same extent as in 1. In most balances, however, it shows somewhat less on the index.
“3. The balance is accurately adjusted should it be necessary to establish a perfect equilibrium between the scales by loading the one with a minute portion of tinfoil (this tinfoil must be left remaining upon the scale during the experiment); both scales are then equally loaded, say with about fifty grammes each, and if necessary the balance is again adjusted (by the addition of small weights, &c.). The load of the two scales is then interchanged, so as to transfer that of the right scale to the left, and vice versâ. A balance with perfectly equal arms must maintain its absolute equilibrium upon the interchange of the weights of the two scales.