522 EEPOET — 1886. 



the whole from one end of the balance-beam. On each side of the hook, and 

 equally distant from it, two slender rods of brass are screwed into the wooden bar,, 

 which support two small platinum cups for holding mercury or dilute acid. The 

 position of these cups is so adjusted that when the whole hangs freely the cups are 

 in Une with the terminal knife-edge of the balance-beam, and have their edges just 

 slightly above its level. The free ends of the insulated wire surrounding the disc, 

 after being firmly tied together for a considerable length and suitably bent, are 

 soldered to the brass supports of the platinum cups, which thus serve as electrodes 

 by means of which a current may be sent through the suspended coil. A precisely 

 similar coil is suspended from the other end of the balance-beam. 



We now come to the arrangement by means of which a current is led through 

 the suspended coils, so as to interfere as little as possible with the sensibility of the 

 balance. This constitutes the essential peculiarity of the instrument, and is effectedl 

 in the following way: — An insulated copper vnre, having its ends tipped with 

 short lengths of platinum, is run along the lower edge of the beam, and is firmly 

 lashed to it by well-rosined silk thread. The ends of this wire, bent twice at right 

 angles, are so placed that their platinum tips dip vertically into one of each pair_ of 

 the platinum cups which are attached to the vertical rods of the suspended coils. 

 From the other cup of each pair proceed two similarly tipped copper wires, which 

 run along the upper edge of the beam, and are also firmly tied to it. These wires, 

 however, only proceed as far as the middle of the beam, where they are bent, first 

 outwards, one on each side of the beam, at right angles to it, and then downwards, 

 so that the platinum tips are vertical. The latter dip into two platinum cups 

 attached to two vertical rods, which spring from the base-board of the balance. 

 These rods are placed at equal distances on each side of the beam, and are of such 

 length that the platinum cups are in line with the central knife-edge of the beam, 

 and have their edges just a little above its level. There are thus in all six cups and 

 six dipping wires. Three of these are in line on one side of the beam, and three on 

 the other. Also the line joining the points of each pair of dipping wires is made to 

 coincide with the corresponding knife-edge ; and, further, the edges of the cups are 

 all in the same plane when the balance is in equilibrium. 



From this it wiU be obvious that any motion of the beam in the act of weighing- 

 causes only a very slight motion of the platinum wires, which dip into the fluid 

 contained in the cups. The resistance, due to the viscosity of the fluid, is thus very 

 small, even in the case of mercury, and much smaller when dilute acid is used. 

 In point of fact, the diminution of sensibility due to this cause is less than in the 

 case of determining the specific gravity of solids by weighing in water in the 

 ordinary way. With clean mercury it is quite easy to weigh accurately to a 

 milligramme. 



The fluxed coils, constituting two pairs, have the same diameter as the suspended 

 coils, and, like them, are made of single turns of insulated wire wound round the 

 edges of circular discs of glass or brass. The discs of each pair are fixed, at the 

 requisite distance apart, to a cylindrical block of wood, so as to have their planes 

 exactly parallel and their centres in the same straight line. To ensure this they 

 are turned up and finished on the same cylindrical block on which they are finally 

 to rest. When in position they are so placed that, when the balance is in equi- 

 librium, each suspended coil hangs perfectly free to move with its plane horizontal 

 and exactly midway between a pair of fixed coils. For this purpose, as will be 

 seen, it is necessary that two large holes be drilled in the upper disc of each pair, so 

 as to allow the brass pillars of the corresponding annular disc to pass freely through. 

 When the connections are made, the current is led through the entire apparatus in 

 such a way that, while the electro -magnetic force acting on the one suspended coil 

 causes it to descend, the electro-magnetic force acting upon the other causes it to 

 ascend. The total force tending to disturb the equilibrium of the balance is thus 

 exactly four times that due to an equal current circulating in two parallel circles of 

 the same diameter and with their planes at the same distance apart. The current- 

 strength is estimated from the number of grammes required to restore the balance 

 to equilibrimn, the weights being placed in small scale-pans attached to the movable 

 part of the apparatus. 



