797 



BALANCE. 



BALANCE. 



798 



other. There are other methods of determining the position of the 

 beams, such as a graduated arc attached to one end of the beam viewed 

 through a compound microscope with a horizontal wire in the focus 

 of the eye-glass, or by a mirror attached to the beam in which the 

 reflected image of a scale is viewed through a telescope. 



All that precedes has reference to tit theoretical construction of the 

 instrument, and supposes that a perfedUbalance can be made, so that 

 equal weights shall counterpoise each other. This is certainly impos- 

 sible in practice, though one balance may be nearer than another. 

 The following simple method, imagined by Borda, overcomes this diffi- 

 culty, provided only the balance be sensible and very well constructed 

 at the knife-edge. Instead of weighing, say a portion of a salt against 

 brass weights, weigh both the salt and the weights against some third 

 substance, say, for distinction, iron weights, as follows : call the 

 scales A and B ; weigh the salt in A against iron weights in B till there 

 is a counterpoise : then remove the salt and substitute the brass 

 weights in A until there is again a counterpoise. It is now of no con- 

 sequence whether the weight of iron was equal to that of the salt or 

 not ; the weight of the salt and of the brass must be the same, because, 

 under the same circumstances, the two counterpoise the same weight 

 of iron. 



Dr. Faraday, in his ' Chemical Manipulation,' sect, ii., gives a number 

 of minute directions on the management of the balance, weighing, &c., 

 to which we refer the student ; but we may here select a few details of 

 general interest. 



Scientific chemistry may be said to owe its very existence to good 

 balances and accurate weighing. Two or three balances are required 

 in the laboratory ; a coarse balance for weighing large quantities, which 

 would injure those intended to show minute differences in small weights, 

 by bending the beam or changing the points of support. This balance 

 should weigh from 1 oz. up to 3 or 4 Ibs., or more, and when fully 

 loaded should turn with 2 or 3 grains. A second balance should weigh 

 from I a grain to 2 or 3 oz., and turn with about i or of a grain when 

 fully loaded ; but the best instrument should be capable of weighing 

 from 600 to 1000 grains and downwards, and indicate with distinctness 

 and certainty differences equal to the 5^55 or gfjg^ part of the weight 

 in the scale. The weights required for these balances will vary from 

 3 Ibs. down to the 100th part of a grain, and form at least two sets; 

 the one consisting of averdupois pounds, ounces, drachms, and the 

 other of grains, from 1000 down to the smallest fractions. These 

 weights are usually constructed in sets, containing as few as possible 

 in each ; thus, four weights of 1, 2, 3, and 4 grains are sufficient to weigh 

 from 1 to 10 grains; and with similar weights of 10, 20, 30, 40, 100, 

 200, 300, and 400, as much as 1100 grains can be weighed. A perfect 

 set of weights, from 500 grains down to the hundredths of a grain, 

 should be kept with the best balance. The weights are commonly made 

 of brass, but the smaller ones are of platinum, which is very desirable 

 on account of its unchangeable nature, and the ease with which it may 

 be cleaned without loss, either by wiping, or momentary exposure to 

 the flame of a spirit-lamp. 



The vibrations of the balance vary with the load, and as this is 

 increased the slower are the vibrations. To a practised eye, the extent 

 and velocity of vibration indicate nearly the weight required to produce 

 equilibrium. The weight required to produce a certain inclination of 

 the beam, or to turn it, should be known when the balance is slightly 

 as well as heavily loaded. A good balance should turn with the 100th 

 of a grain when 600 grains are in each scale, or with jgjgj of the weight 

 to be estimated. 



When a balance is overloaded, it is liable to let ; that is, the side 

 which is lowest becomes permanently depressed. For example, a balance 

 with equal weights in each pan, too heavy for its size, if placed hori- 

 zontal will remain so ; but if one of the pans be a little depressed, it 

 will continue to descend with accelerated force just as if it were the 

 heavier of the two. This effect is more likely to take place in a delicate 

 balance, when overloaded, than in a coarser one. The setting is 

 brought about by the position of the fulcrum below the line which 

 joins the extreme points of suspension of the beam, whereby the centre 

 of gravity of the beam is brought below the fulcrum. 



A good method of testing a balance is to remove the pans, and to set 

 the beam vibrating, which it should do in a regular and equable manner, 

 and gradually take up a horizontal position. It should then be reversed 

 on its stand and made to oscillate as before, when, should it again come 

 to rest in a horizontal position, the balance is good ; for this test is a 

 severe one, generally disclosing, as it does, some defect in the work 

 of the middle knife-edge, and the planes on which it rests. When the 

 pans are hung upon the beam the balance should still remain horizontal, 

 but the accuracy of the lines of suspension should be tried by changing 

 the pans, then by reversing the beam, and afterwards by changing the 

 pans again. Then put equal weights into the pans, and change them 

 from one pan to another; if the equilibrium be now retained, the 

 lengths of the arms are equal. 



The balance was well represented in the Great Exhibition of 1851 ; 

 so much so as to call for a liberal encomium in the ' Jury Report, 

 Class X. " When it is considered, in order to have a balance as perfect 

 as powible, how very many circumstances are to be attended to, it 

 must be deemed highly honorable to the exhibitors of good balances to 

 have produced instruments standing the test of the rigorous exami- 

 nation to which they have been subjected." 



Among the balances exhibited were some novelties. First, with respect 

 a the modes of suspension. In Mr. Fox's balance, the beam has pivots, 

 .he conical ends of which play in hollow agate cones of larger angle.* 

 .n the short-armed balance, constructed many years ago by Captain 

 iater and Mr. Robinson, the then novel feature was the long bearing 

 on which the pans were suspended, instead of the hook and eye 

 arrangement employed on the continent, and in the balances imported 

 into this country. The defects of this arrangement are, that the hook 

 does not always occupy the same position upon the ring with reference 

 io the centre of motion, so that the beam is practically sometimes 

 onger and sometimes shorter, causing differences in the indications of 

 as much as '01 of a grain. Secondly, the divisions of the scale to which 

 the index of the balance points are numbered 1, 2, 3, &e., to the right 

 and left of zero, to which the index points when the beam is horizontal. 

 [t is suggested that the number of the divisions should run the same 

 way, and not begin with zero ; it being better to number that division 

 xi which the index points when the beam is horizontal, 10, 20, or some 

 multiple of 10, in order to avoid the liability to error consequent upon 

 [laving positive and negative numbers. Thirdly, in the balances exhi- 

 bited by Mr. Oertling (a London artist, well known to scientific men 

 for the excellence of his instruments), the beam was in one case coated 

 with platinum, and in another case with palladium : the operation having 

 been performed by Mr. Henry, by some electro-metallurgic process. 

 In Mr. Oertling's large balance, the beam was 3 feet in length, and 

 capable of carrying 56 Ibs. in each pan. The beam was a pierced rhomb, 

 constructed so as to give great strength in proportion to its weight, 

 any tendency to lateral yielding being counteracted by edge bars. The 

 three knife edges are long, and the whole length of the middle one 

 rests on a plane surface of steel. Plane surfaces of steel also rest on 

 the extreme knife-edges for suspending the scale pans. In the second 

 balance, the palladium coated beam is 16 inches long; the three knife- 

 edges, as well as the plane on which the middle one rests, and the 

 planes from which the pans are suspended, are of agate, so that the 

 instrument is not liable to be affected by the acid vapours of the 

 laboratory. In this instrument the beam is graduated, so that small 

 differences of weight can be determined by placing a small platinum 

 wire weight on one of the divisions of the graduated beam. Mr. Marriott 

 exhibited a chemical balance, the beam of which was made of a wide 

 piece of fir-wood, with interstices cut out, so as to leave a strongly- 

 framed net-work; it was said to be sensible to the 100th of a grain. 

 In Mr. Dover's balance, constructed on the plan of Robinson's, the final 

 adjustments in the direction of the length of the beam, and in a 

 direction perpendicular to it, are effected by a cut at each end of the 

 beam, making an angle of about 45 with the axis of the beam, and 

 capable of being widened by means of a screw. In the American 

 balance, capable of carrying 56 Ibs. in each pan, the knife-edges are 

 square bars of steel, each fitted into a . socket attached to the beam, 

 having a rectangular notch, so that any one of the four edges of the 

 bar may be used as a knife-edge, which would be found useful in case 

 of one of the edges being damaged. The socket in which one of the 

 extreme knife-edges is fixed moves in a slit in the direction of the 

 length of the beam, and is adjusted in that direction by means of two 

 screws. Mr. A. Oertling (Prussia) had a balance in which the knife- 

 edges were let into dove-tail notches in the beam. The adjustment of 

 the distance of the extreme knife-edge from the middle knife-edge was 

 effected by means of a vertical cut in the metal of the beam, capable of 

 being slightly widened or contracted by screws. Fourthly, in some of 

 the French balances the pans were of platinum, suspended by silver 

 wires. Fifthly, there were various contrivances for checking the oscil- 

 lation of the beam and pans when in action, but none of them were 

 considered by the Jury to be equal to Wollaston's arrangement. In 

 Dolberg's balance (Mecklenburg Schwerin) the oscillation of the pans is 

 checked by hair-brushes which, on turning a handle, ascend, until the 

 ends of the brushes touch the under side of the pan. In Mr. Fox's 

 balance, already mentioned, the attraction of a magnet brings the beam 

 exactly to zero before weighing. 



In the measures taken to restore the standards of weight and 

 measure, to replace those which had been destroyed by the burning of 

 the Houses of Parliament, the first step in the process of arriving at 

 the weight of the lost standard was to compare among themselves the 

 different troy pounds with which the lost standard had been compared 

 by Captain Kater and Captain V. Nehus. The balance used for these 

 comparisons was furnished by Mr. Barrow, on Robinson's principle, and 

 is described by Professor W. H. Miller, in his paper in the ' Philosophical 

 Transactions ' for 1856, ' On the Construction of the New Imperial 

 Standard Pounds,' &c. " The beam is made sufficiently strong to carry 

 a kilogramme in each pan. The middle knife-edge is about 1'93 inch 

 long, and rests, when the balance is in action, throughout its whole 

 length on a single plane surface of quartz. The surfaces of quartz 

 which rest upon the extreme knife-edges, and from which the pans are 



* In the balanco constructed under the direction of Gauss and Weber (' Got- 

 tengen Transactions '), the beam is suspended by two watch-springs, and each 

 pan by a single watch-spring. In Steinhcil's balance tbe beam and pans are 

 suspended by wires or silk ribbons : or by another method the beam carries two 

 small steel spheres in the middle, resting upon a steel plane, while at each end 

 of the beam is a sphere, upon which rests either the plane or the slightly con- 

 cave spherUal surface of the plate from which the scale-pan is suspended. 

 None of these balances were exhibited. 



