Erg per Hour per Square Millimeter [erg/(h·mm²)]

The unit erg per hour per square millimeter, written as erg/(h·mm²), measures energy flux density or power per unit area using the cgs system. An erg is defined as the energy required to move 1 dyne over 1 centimeter, or 1 erg = 10⁻⁷ joules. Dividing by hour and mm² provides the rate of energy transfer per small area over time.

This unit is typically used in laboratory-scale physics, radiation experiments, and surface energy studies where small areas and low energy transfers are involved. Applications include:

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  Measuring radiation incident on tiny surfaces

  
  
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  Calorimetry experiments on small samples

  
  
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  Micro-scale energy transfer studies in materials science

  
  

Heat or energy flux can be expressed mathematically as:

where q is energy flux in erg/(h·mm²), E is energy in ergs, A is area in mm², and t is time in hours.

Conversion to SI units:

While largely of historical or specialized use, erg/(h·mm²) provides a convenient cgs-based measure for micro-scale, low-intensity energy transfer in scientific experiments.

Calorie (th) per Minute per Square Centimeter [cal(th)/(min·cm²)]

The unit calorie (th) per minute per square centimeter, written as cal(th)/(min·cm²), measures heat flux density or thermal power per unit area using the thermochemical calorie (1 cal(th) = 4.184 J). It represents the amount of heat energy transferred through 1 cm² of surface every minute.

This unit is often used in laboratory experiments, solar energy studies, and material testing, particularly when small surfaces and longer exposure times are involved. Applications include:

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  Solar radiation measurements on small panels or surfaces

  
  
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  Thermal testing of materials over extended periods

  
  
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  Calorimetry experiments on small samples

  
  

Mathematically, the heat flux is expressed as:

where q is heat flux in cal(th)/(min·cm²), Q is energy in calories, A is area in cm², and t is time in minutes.

Conversion to SI units:

Although SI units such as W/m² are standard, cal(th)/(min·cm²) remains useful in historical calorimetry, laboratory experiments, and solar radiation studies, providing an intuitive measure of energy transfer per minute per small surface area.