MEASUREMENTS 37 



ing the item of unknown mass and the standard masses on opposite sides 

 of the balance point is that the two arms of the beam may not be identi- 

 cal. This possibility can be avoided by comparing the unknown and 

 standard masses on the same arm against some inert material on the 

 opposite arm. In following such a procedure, weighing would be too 

 cumbersome, but some of the new balances operate on this principle. A 

 full set of standard weights is hung on one arm of the balance, counter- 

 acted by an equal mass on the opposite arm. The unknown material is 

 placed on the same arm as the standards, and then standards are removed 

 in various combinations until balance is restored. Recording the values of 

 the standard weights removed gives the mass of the unknown object. 

 These new balances fit into our mechanical world because the standard 

 weights are removed by a system of hooks, arms, and gears, and this 

 mechanical system is coupled to a dial which shows the mass of the 

 unknown object directly. Those of us who grew up with the equal-arm 

 analytical balance, however, are still a little suspicious of anything this 

 easy. 



Time: The units in which the scientist expresses time are the everyday 

 units: seconds, minutes, etc. The second is the standard unit, defined 

 formerly as 1/86,400 of a mean solar day, but since the international 

 agreement in 1960 as 1/31,556,925.9747 of the year 1900. This is prob- 

 ably a temporary standard, since it is likely that the period of vibration 

 of some simple molecule will be adopted as a new fundamental standard 

 of time. The new standard second is necessary for some purposes, as in 

 astronomy, but for most practical purposes clocks are still good enough. 



Time is an especially important unit in biology because living material 

 is always changing, and measurement of rates gives important informa- 

 tion about mechanisms. Time can be measured with a clock or stop- 

 watch, but where the human reaction time can contribute to errors 

 some electrical or mechanical device may be used to start and stop the 

 clock. Some exceedingly fast reactions are followed and timed by com- 

 plex electronic equipment. 



Energy: Of all the quantities that must be measured, the abstract 

 collection known as energy is probably the most difficult to understand. 

 Energy is commonly defined as the capacity or ability to do work, but 

 work itself is of several kinds. Energy can be converted from one form 

 to another, and the mathematical expressions denoting these transforma- 

 tions can become quite complicated. 



The term "heat" means many things to many people, but here we 

 shall refer only to the quantity of thermal energy which a body contains. 



