MEASUREMENTS OF THE INTENSITY OF SOLAR 

 RADIATION. 



By H. E. Wood, M.Sc, F.R.]\Iet.S. 



The Transvaal Observatory possesses two instruments for the 

 study of Solar Radiation — an Angstrom Compensating Pyrhelio- 

 meter and a Callendar Electrical Sunshine Recorder. 



o 



In the Angstr5m Pyrheliometer the principal part of the instru- 

 ment consists of two similar strips of blackened platinum. These 

 are so arranged that, whilst one of them is exposed to the solar 

 radiation, the other may be placed in circuit with a battery and a 

 variable resistance, and have a current of adjustable strength 

 sent through it. A small thermo-couple is placed at the back of 

 each platinum strip near the centre, and the two couples are 

 connected up to a sensitive galvanometer in such a wa^' that when 

 they are heated they tend to send currents through the galvano- 

 meter in opposite directions. If the sun is allowed to shine upon 

 one strip, A, the strip becomes heated by the absorption of the 

 solar radiation, the thermo-couple placed at the back of it becomes 

 affected and a thermo-current is sent through the galvanometer, 

 producing a deflection. The companion strip B, which is shaded 

 from the sun, has now a current of electricity sent through it. 

 The strip B becomes thereby warmed, and consequently a second 

 thermo-current passes through the galvanometer in the opposite 

 direction to that of the first. The strength of the current passing 

 through the strip B is then varied until the galvanometer shows 

 no deflection at all. This being so, the two thermo-currents must 

 have exactly the same strength, and accordingly the tw^o strips 

 of blackened platinum must both be in the same thermal state. 

 The inference, then, is that the amount of energy absorbed by the 

 strip A, which is exposed to the solar radiation, is equal to that 

 developed in the strip B, by the passage of the electric current 

 through it. Now the energy developed in the strip is equal to 

 Kri-, where A' is a constant quantity, r the resistance of the strip 

 and i the strength of the current. 



Hence the amount of solar radiation absorbed by the strip can 

 be determined from the strength of the electric current^necessary 

 to produce a balance, and certain known properties of the strip. 

 In the actual" apparatus the strips ATand B are interchangeable, 



