136 



RADIATION BIOLOGY 



the work done when a force of 1 newton acts through a distance of 1 m. 

 The newton is the force required to give a 1-kg mass an acceleration of 

 1 m/sec^. The joule is equivalent to IC ergs. The gram-calorie, or 

 small calorie, is a unit of heat which is the energy (heat) required to raise 

 1 g of water 1°C in the interval from 15° to 16°C. The watt is the mks 

 unit of power and is equivalent to 1 joule/sec or 10^ ergs/sec. The 

 microwatt is equal to 10 ergs/sec. 



RADIOMETRIC UNITS 



The conversion factors for the various units of energy, power, and 

 intensity presented in Table 3-6 are arranged in the order of increasing 

 size for convenience in comparison. Since the mks system has many 



Table 3-7. Multiplying Prefixes for Quantitative Physical Units 



Prefix 



mega- 

 kilo-, 

 hecto- 

 deka- 

 deci- . 

 centi- 

 milli- 

 micrO' 

 nano- 

 pico-. 



Example 



megacycle 



kilogram 



hectowatt 



dekaliter 



deciliter 



centimeter 



millimeter 



microgram 



nanogram 



picogram 



practical advantages for general work and since the Bureau of Standards 

 calibrates its radiation standard sources in microwatts per square centi- 

 meter of irradiance at a specified source distance, the mks system is con- 

 sidered preferable for radiant-energy measurement. Thus the microjoule 

 may be used in place of the erg (1 mJ = 10 ergs), and the microwatt per 

 square centimeter in place of the erg per second per square centimeter. 

 It will be noted that the watt per square meter is equivalent to the micro- 

 watt per square millimeter and to 100 /xw cm~^ The U.S. Weather 

 Bureau, probably for historical reasons, still uses the calorie per minute 

 per square centimeter for the specification of solar irradiance, but the 

 watt per square meter is now often preferred. The multiplying factors 

 for the various prefixes used with physical units are given in Table 3-7. 

 The prefixes "nano-" and "pico-" have been recommended by the sym- 

 bols committee of the Royal Society of London (Pirie, 1951). 



PHOTOMETRIC UNITS 



Units of illuminance (Table 3-6) are based on the international candle 

 as the unit of luminous intensity (Illuminating Engineering Society, 1952; 



