230 CHAP. XXV. PHOTOSYNTHETIC STORAGE OF SOLAR ENERGY 



unobstructed light to pass through, the full intensity, I, 

 being measured by the Radiometer or the Calorimeter. 

 When b is interposed, the plant absorbs a certain amount 

 of light, and the transmitted radiation, T, is measured as in 

 the last case. The radiation absorbed is found from the 

 difference between I and T. 



Magnetic Radiometer. — This has already been fully 

 described ; it is diagrammatically represented here as r, 

 placed opposite to the Bubbler. The full radiation, I, 

 produces a deflection D of the Radiometer, whereas the 

 radiation, T, transmitted through the plant causes a reduced 

 radiometric deflection d. 



The Calorimeter. — An independent measurement is made 

 by the Calorimeter c, mounted on a sliding stand s ; the 

 slide is pushed along the guide-rail, till the Calorimeter c 

 intercepts the light passed through the two Bubblers, one 

 after the other. The incident radiation I and the transmitted 

 radiation T are measured by the rise of temperature of the 

 water in the Calorimeter. The temperature is accurately 

 measured by a thermo-electric couple in circuit with a 

 sensitive Galvanometer, the deflections of which measure the 

 incident radiation I and the transmitted radiation T. 



Measurement of energy by the Calorimeter. — The size of 

 the calorimetric vessel, made of thin sheet silver, is 

 1-5 x 2 X 2 cm. It is highly polished on three sides to 

 diminish loss by radiation. The front surface is coated with 

 lamp-black for the absorption of the incident light. The 

 weight of the Calorimeter is 2-y grms., the water-equivalent 

 being 2-7 X 0-06 = 0-16 grm. The quantity of water in 

 the Calorimeter is always kept the same, namely 5 grms. 



Thermo-electric couple for measurement of temperature. — 

 The energy of the incident radiation and that of the trans- 

 mitted radiation are directly measured from the number of 

 heat-units produced in the Calorimeter. This is accurately 

 determined from the rise of temperature of the contained 

 water by means of a sensitive thermo-electric couple made 

 of fine copper and nickel wires. One junction t' is placed in 



