62 LECTURES TO SCIENCE TEACHERS. 



balance is contained in Appendix X. to the Ninth Annual 

 Report of the Warden of the Standards. 



The principal objection that first occurs to this method is 

 that, as it requires the observer to stand close to one side of 

 the balance, there is a tendency to the temperature of the 

 beam becoming unequally affected by the greater heat of the 

 observer's body, and thus to cause discordances in the results 

 of successive weighings. But, on the other hand, the shorter 

 the beam, the less risk there is of its being so acted on. 



The mode of observing the oscillations of the longer-armed 

 balance is not open to this objection. The observer is seated 

 about five feet from the balance case, and views the move- 

 ment of the pointer over the index through a telescope. Any 

 difference of one-tenth of a division is thus readily observed 

 by estimation. 



One great advantage of this short-armed balance is that, 

 being so small, and placed as it is upon a metallic plate, 

 it can be covered with an ordinary glass bell receiver, and 

 used as a vacuum balance. In weighings in a vacuum it 

 is very important to be able to exhaust the air rapidly and 

 frequently ; and that the balance-case serving as a receiver 

 should be as small as possible, so as to reduce to a minimum 

 the volume of air to be exhausted. 



18. The pans of a balance should be suspended in such 

 a manner that in all positions the corresponding rods or 

 chains of the two pans may be parallel to one another ; else 

 the weights, though equal, will not be in equilibrium. 



In weighings with balances of precision, the degree of pre- 

 ponderance of either pan is indicated by a needle or pointer 

 fixed to the beam, either at its centre, in a line perpendicular 

 to the axis of the beam and pointing downwards, or at either 

 end and in continuation of its axis. In both cases the 

 pointer moves along a graduated scale. But an index pointer 

 placed perpendicularly to the beam affects its equilibrium 

 when turned from its horizontal position ; the measure of the 

 momentum of the pointer being its weight multiplied by the 

 distance of its centre of gravity from the vertical line. The 

 error thence arising may, however, be corrected by continuing 

 the pointer, or counterpoising it, on the opposite side of the 

 beam. 



The finest balances of the Standards Department have the 

 index pointer at each end of the beam, as shown in Fig. 1. 



