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tions begin from zero on the extreme left and have numbers increasing towards the right. A carriage is moved along the graduated bar to any required position by a sliding piece controlled by a cord which can be pulled from either end, and this carriage, by itself or with an additional weight, forms the movable weight referred to above. The position of the carriage is indicated by a pointer which moves along the lower scale. Each additional weight has in it a small hole and slot which pass over conical pins in the carriage. This ensures that the weight is always placed in a definite position. The balancing weight is moved along the beam by means of a self-releasing pendant carried by the sliding piece above referred to. To this pendant is attached a vertical arm (seen in the figure) which passes up through the recess in the front of the weight and carriage and so enables the carriage to be moved with the sliding piece. The stationary weight is placed in the trough shown at the right-hand end of the instrument. The trough is V shaped, and the weight cylindrical, with a cross pin which passes through a hole in the bottom of the trough. The weight is thus placed in a perfectly definite position and always has the same leverage. It is so chosen as just to keep the beam in the sighted position when the sliding weight is at the zero of the scale.

Since the mutual action of the rings is to bring the beam towards the sighted position when displaced by the weights, and the equilibrating couple is that due to the displacement of the sliding weight from zero, the latter couple increases as the current increases, and hence motion of the sliding weight towards the right corresponds to increasing currents. The use of the stationary weight gives a scale of double the length which would be obtained without it.

In the top of the lower or finely graduated scale are notches which correspond to the exact integral divisions in the upper fixed scale. Thus the reading in the fixed scale is got when the pointer is at a notch, without error from parallax due to the position of the eye. The reading when the pointer is between two notches is easily obtained by inspection and estimation with sufficient accuracy for most practical purposes. When however the greatest accuracy is required, the reading is taken on the lower scale, with the aid of a lens, and the current strength calculated from the table of doubled square roots given in the Appendix below.

Four pairs of weights are given with each instrument. Of these one set is for the sliding platform, the other set are the corresponding counterpoises. The weights of each set are in the ratios 1: 4: 16: 64, and are so adjusted that, when the carriage is placed with its index at a division of the inspectional scale, the instrument shows a current of an integral number of amperes, half-amperes, or quarter-amperes, or some decimal subdivision multiple of one of these units of current.

or

The accurate adjustment of the zero is effected by a small metal flag as in a chemical balance. This flag is set in any required position by means of a fork moved by a handle beneath and outside the case of the instrument. The sliding weight is brought to zero with the corresponding counterpoise in the trough, and then the flag is turned to one side or the other until the pointer of the beam (seen on the extreme right and left in Fig. 13) is just at

zero.

When necessary for transit or otherwise, the beam in the centi-ampere and deci-ampere balances is lifted off its supporting ligament by turning an eccentric by a

[graphic][subsumed]

FIG. 14.-STANDARD DEKA-AMPERE BALANCE.

shaft under the sole-plate of the instrument. In the other balances the beam is fixed for carriage by placing distance pieces between the upper and lower parts of the trunnions and screwing them together by milled headed screws kept always in position for the purpose.

Fig. 14 shows the standard deka-ampere balance for the measurement of currents ranging from 1 to 100 amperes. The only essential difference in construction in this instrument consists in the use of a small number of turns of thick copper-wire for the rings, more massive connections to carry the current to and from the movable rings, and special electrodes, so that the instrument can be placed in an electric-light current without perceptibly increasing its resistance.

In this balance (and in the similar hekto-ampere balance adapted for currents from 6 to 600 amperes), when made so as to measure alternating as well as continuous currents, the current is carried by a twisted rope of copper wires, each of which is insulated from its neighbours along its length. The object of this arrangement is to prevent the distribution of an alternating current over the cross-section of the conductor from being affected by inductive action.

Fig. 15 shows the kilo-ampere balance, for currents varying from 25 to 2,500 amperes. Of this we have The details are

already described the main parts. similar to those of the other balances, and will be easily made out from the cut. The rings are massive, and the lower fixed ring, shown in the figure as having several turns, is made of bare copper strip, the different turns of which are kept apart by slips of mica.

The centi-ampere balance may be used as a voltmeter

[graphic]

FIG. 15.-STANDARD KILO-AMPERE BALANCE.

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