66 INTENSITY OF SOLAR RADIATION. 



SO that the effect of any slight differences there may be between 

 them are eliminated. 



The details of a determinat'on of the solar radiation are as 

 follows : — 



IQOS. April 3rd. Standard Mean time at beginning of experi- 

 ment II hours 30 minutes. 

 Strip A exposed to sun : current through B for no deflection. 



0*3060 amperes. 

 Strip B exposed to sun : current through A for no deflection,. 



0*3063 amperes. 

 Strip B exposed to sun : current through A for no deflection, 



0*3072 amperes. 

 Strip A exposed to sun : current through B for no deflection,. 



0*3060 amperes, 

 ^lean value of current = 0*3064 amperes, 

 ^lean constant for strips = i6*o. 

 Intensity of solar radiation = = Ki-' 



= 16.0 X (0*3064)"-' 

 = 1*502 gram-calories per sq- 

 cm. j)er minute. 



f From a series of observations made with this instrument between 

 July ist. 1907, and June 30th, 1908. the value of 1.36 gram-calories 

 per square centimetre per minute was obtained for the solar radia- 

 tion at Johannesburg (26 degrees 11 minutes South ; altitude 

 5,925 feet). The mean radiation for the two mid-winter months, . 

 June and July, was found to be 1.30 gram-calories per sq. cm. per 

 minute and for the mid-summer months. December-January, 

 1.45 gram-calories p)er sq. cm. per minute. 



The intensity of solar radiation at the extreme limit of the earth's 

 atmosphere has been estimated to be 3 gram-calories per sq. cm., 

 per minute. 



In the case of the Callendar Electrical Sunshine Recorder, the 

 intensity of solar radiation is measured by exposing two flattened 

 spirals of platinum wire to the rays of the sun. One spiral is 

 coated with some highly absorbing medium while the other is 

 left bright. The result is that on exposure to the sun there is 

 a dift'erence between the electrical resistances of the two coils, 

 just as when a black bulb and a bright bulb thermometer are 

 exposed to the sun, different readings are obtained. The difference 

 between the resistances of the two coils is automatically com- 

 pensated by the movement of a slider along a bridge wire. A pen 

 attached to the slider then registers tlTe changes in the intensity 

 of the solar radiation. The measurements obtained are standardised 



by means of the Angstrom Pyrheliometer. 



As the Callendar instrument automatically records the variations 

 of solar radiation from minute to minute throughout the day. a 

 measurement of the total daily amount of radiation can be obtained 

 from it. In the following table are given the mean monthly values 

 of the maximum daily radiation and the total daily radiation ; 

 the largest maximum radiation of any day and the largest total 

 radiation on anyj^day. 



