32 MEASUREMENTS 



necessary for the biologist to develop some other kind of units. An anti- 

 biotic, for example, might be a mixture of several different chemical 

 compounds. The effectiveness of the antibiotic in preventing the grou^th 

 of bacteria depends upon the source of the preparation and upon the 

 proportion of the different compounds in the mixture. As merely refer- 

 ring to a certain number of milligrams of the substance does not convey 

 adequate information, some unit which expresses the activity and can 

 be based upon a standardized laboratory test is much more useful. In- 

 stead of an absolute amount of material w^e refer to an amount which 

 will cause a certain effect. Numerous units of this sort have been used 

 in experimentation in biology because of the variability and complexity 

 of biological materials. 



Direct Measurements and Null Measurements: The simplest method 

 of comparing one object with another is to place them side by side. 

 Units of length are commonly measured in this way. A scale for weigh- 

 ing objects could be constructed from a simple steel spring. The heavier 

 an object is, the more the spring will be deflected. An electrical quantity 

 such as voltage can cause a certain deflection of the needle in a meter. 

 The weight measured by the spring and the voltage measured by the 

 meter are examples of determinations of the effect caused by the phenom- 

 enon being measured. Many kinds of measurements are as simple and 

 direct as these, but in certain instances it is advantageous to make a 

 "null" measurement. Instead of measuring a value direcdy we measure 

 that amount of force required to oppose it. A balance used to measure 

 mass consists of a pair of pans suspended on opposite ends of a f)eam; 

 the material we wish to weigh exerts a force, and we add weights to 

 counteract this force. It is easier to make precise weights than precise 

 springs. 



Electrical quantities can be measured by means of a Wheatstone 

 bridge, as shown in Fig. 4-1. A pair of matched resistors is connected, 

 as shown, with another pair of resistors, and a voltage is applied from the 

 battery. If the resistance of Rs is greater than Ri, a current in one direc- 

 tion is indicated by the meter; or if R4 is greater than Rs, current will 

 flow in the opposite direction. The resistor Rs can be adjusted until 

 there is no current so that we can measure R4 in terms of R3. Because 

 voltage, current, and resistance are precisely interrelated by Ohm's law, 

 the basic bridge circuit can be adapted to a variety of measurements 

 where a third "unknown" value is calculated from the other two. Greater 

 precision is available here also because it is easier to make and evaluate 

 resistors than meters. 



