COMPENSATION PENDULUM. As the length of a rod or bar of any material depends on its tem perature (see BEAT), a clock with an ordinary pendulum goes faster in cold and slower in hot weather. In the method of correction usually employed, and called a d va n t ge is taken of the fact that different substances have different eoeflivients of linear expansion: so that if the bob of the pendulum is so suspended as to be raked by the expansion of one substance. and depressed by the expansion of another, the lengths of the effective portions of these substances may be so adjusted that the raising and depression. taking p 1 a ee simultaneously. may leave the po,ition of the bob unaffected. There are two common methods of effecting this. differing a little in con struction. but ultimately de pending on the same principle.
Of these, the »icrcarbil pond u him is ihe more easily de scribed. The rod AC and the framework CB are of steel.
inside the framework is placed a cylindrical glass jar.
nearly full of which can be raised or depressed by Homing a screw. With an inerease of temperature. the steel portion AC is lengthened by an amount proportioned to its length, its eoellicient of linear dilatation, and the change of temperature. conjointly—and tints the jat• of mercury is re moved from the axis of suspension. Mit neglect ing the expansion of the glass, which is very small, the mercury rises in the jar by an amount proportional to its bulk. its coefficient of cubical expansion, and the change of temperature con jointly. Now, by increasing or diminishing the quantity of mercury, it is obvious that we may so adjust the instrument that the length of the univalent simple pendulum shall be unaltered by the change of temperature, whatever be its amount, so long as it is not great enough to change sensibly the coefficients of dilatation of the too metals.
The construction of the gridiron pendulum will be easily understood from the cut. The shad ed bars are steel; the second and fourth ones arc zinc, or some substance whose coefficient of linear exp:11/4011 is considerably greater than that of steel. It is obvious from the figure that
the horizontal bars are merely con nectors, and that their expansion has nothing to do with the vibra tion of the pendulum. so they may be made of any substance. It is easily 'Well that an increase of tem perature lowers the bob by expand ing the steel rods, whose effective length consists of the sum of the lengths of one of the outer rods and the steel bar to which the bob is attached; while it raises the bob by expanding the zinc bars, whose effective length is that of one of them only, the other, as well as one of the outside steel rods, being added to the instrument for the sake of symmetry, strength, and stiffness only. If the effective lengths of steel and brass he in versely as their respective expan sion coefficients, the position of the bob is unaltered by temperature; and therefore the pendulum will always vibrate in the same period. This is on the supposition that the weight of the framework may be neglected in comparison with that of the bob; if this weight must be taken into account. the requisite adjustments, though possible, are greatly more complex, and can only be alluded to here. Praetieally, it is found that a strip of dry fir-wood, carefully varnished to pre vent the absorption of moisture, and consequent hygrometric alterations of its length, is very little affected by change of temperature; and in many excellent clocks till.: is used as a very effective substitute for the more elaborate forms just de scribed. In astronomical clocks, though they are kept at a temperature as nearly constant as pos sible, the compensation of the pendulum is a matter of delicacy.