66 ELEMENTS OF LABORATORY WORK 



46. New Test for Equal Quantities of Matter. Atwood's 

 Machine. By experiment with Atwood's machine it will be 

 found that an additional mass, placed on one of the equal 

 masses which are connected by the silk thread passing over the 

 pulley, and allowed to fall for a measured time, will have 

 acquired at the end of that time, together with these equal 

 masses, a certain speed. This speed may be measured easily, 

 for, if the additional mass be removed by a ring, the two equal 

 masses continue to move uniformly with the identical speed 

 which the system possessed at the moment of this removal. 

 The uniform motion of the two equal masses after removal 

 of the small mass which produced the acceleration, is a matter 

 of direct observation which is in accordance with previous 

 observations. 



Those masses which, under the same conditions, acquire 

 equal speeds are equal masses. Those masses are equal 

 which, when added for an equal time to this system containing 

 two equal masses connected by a string, produce in it equal 

 speeds. 



By the use of two similar pulleys, each with the same 

 masses connected by a thread, a comparison of the speeds 

 acquired may be made independently of the time taken. Direct 

 observation of coincidence by sight and hearing will decide 

 whether the speeds, and hence the masses, are equal. In 

 this comparative method we are less dependent for accuracy 

 on the smooth running of the pulleys. Provided they run alike, 

 the conditions are the same for each change occurring. 



We shall find later that other stresses, besides that exist- 

 ing between the earth and all bodies removed from its surface, 

 may be utilised for measuring quantity of matter. 



We must not forget that this instrument enables us to 

 surmise the manner in which a body falls to the earth by 

 causing it to fall slowly. This retardation is produced by 

 causing the body to form part of a system of three bodies, 

 two of which are equal in mass and connected by a string 

 so as to be in equilibrium. The speed which this body ac- 

 quires under various conditions is not directly measured, but 

 is taken from the uniform speed with which the system moves 



