222 



THE COMPENSATED PENDULUM. 



Bodies increase in volume with an elevation of temperature 

 and diijiinish when it falls. The pendulum then changes its 

 dimensions with every variation of temperature, and the same 

 is the case with all other parts of the machine. 



The elongation of a body in any one direction by heat, is 

 known as its linear dilatation, and its increase in volume ; that is, 

 in all three directions, is the cubical dilatation ; this depends on 

 its linear dilatation in length, breadth and thickness. 



The result to be obtained in a pendulum by compensation 

 is to so construct the same that the center of oscillation will 

 always be in the same point. It is evident that heat lowers this 

 point and cold raises it, and as we said before, that the time- 

 producing qualities of the pendulum depend on this oscillating 

 point and only by compensation is the desired effect obtained. 



I will show you two of the best methods of producing com- 

 pensation, and begin first by using two metals. The principle 

 underlying this method is the unequal expansion of different 

 metals in the same temperature. This furnishes us with the 

 first step towards compensation. 



Let lis take a steel rod of the length arrived at by calcula- 

 tion, with a nut and screw on the lower end, resting on this nut 

 is a brass collar with a groove cut in the top. Here is a rolled 

 and drawn zinc tube of a calculated length and thickness in pro- 

 portion to the main rod. This zinc tube is drawn on over the 

 main rod and rests on the brass collar at the lower end and at 

 the upper end of the zinc tube, and resting on the same is an 

 iron collar, into which is firmly screwed an iron tube which is 

 slipped on over the zinc tube, and at the lower end of this iron 

 tube is attached the weight or bob. It will be seen that this 

 main rod lengthens with heat, and as it lowers, the zinc tube 

 which surrounds it lowers also but the upper end of the zinc 

 being free and this metal possessing greater linear dilatation, 

 moves upwards on the main rod and with it draws up the iron 

 tube that surrounds the zinc and carries with it the weight or 

 bob. The upward dilatation of the zinc tube is just sufficient 

 to overcome the downward dilatation of the main rod, thus 

 keeping the center of oscillation in the same point. In order to 

 construct a compensated pendulum of this kind it is necessary 



