MEASUREMENTS 



33 



Simple and Derived Units: Measurements of length, mass, and time 

 require simple units: either the basic units of the metric system or frac- 

 tions or multiples of these. Certain other quantities may become exceed- 

 ingly complex in that they involve tw^o or more of the basic units. Force, 

 for example, is mass times acceleration, but acceleration is change in 

 velocity per unit time, while velocity is measured in length units per 

 unit time. Some of the complex measurements necessary in biology are 

 rather difficult to reduce to basic units. 



Fig. 4-1. "Null measurement" of resistance is performed with this Wheatstone 

 bridge. Ri = R2, R3 is adjustable and of known resistance, R4 is unknown. When 

 Rg = R4, zero current is shown on the meter. 



Effects of Measurement upon the System: One of the great difficulties 

 in biological measurement is the response of the living material to the 

 very act of measuring. It is theoretically impossible to measure anything 

 writhout affecting it, but the physicist and chemist have learned that the 

 effects caused by their instruments are small. Living organisms, hou^ever, 

 respond to small changes in environment. Emotional disturbances in 

 animals are well known, but even plants show differences in response 

 upon handling. The experimental biologist must therefore specify the 

 conditions under which measurements were made and be prepared to 

 accept the fact that his figures are not normal. He can only hope that 

 his numbers can be repeated in other measurements under similar con- 

 ditions and that sets of measurements under a variety of conditions will 



