100 



METEOROLOGICAL EOl'IPMENT FOR SHORT WAVE 



20X 



ELEMENT 



+ b(ios vj 



20C^000 



qooo 



FiuuRE 2. Schenuitic circuit of electronic unnililicr. 



The resistance of the thermal element, A', controls the bias of one triode of the double triode, 6SK7, which acts as a vacuum tube voltmeter 

 to compare the resistance of the thermal element with a standard resistance. A 1-ma recording meter s placed between the two plates. The 

 resistance in the grid circuits is so chosen as to place 10 v across the thermal element at the lowest temp3rature of each ran?e. This voltage 

 decreases as the temperature rises. The zero is set by means of a 103-ohm potentiometer in the cathode circuit. Calibration of the amplifier is 

 obtained by switching a series of precision resistors in steps of 1,003 ohms into the circuit in place of the thermal element. A range of roughly 25 C 

 for full scale is used, and changes of 0.25 C can be measured. Sufficient overlapping is provided so that both wet and dry bulbs can record on 

 a single setting. 



The stability is such that with a change in line voltage between 95 and 120 v there is no readable change in the meter deflection at (when the 

 tubes are balanced) or at full scale reading. When tubes are replaced there is, at worst, a change of 1 per cent of full scale deflection tapering to 

 no deflection at 0. 



reliable at very low wind speeds (of the order of 1/2 m 

 per second) and may stop completely. A special ane- 

 mometer for low wind speeds" has been designed by 

 the British Chemical Warfare Service and is used as 

 a regular piece of field ecpiipment by its units. In the 

 United States a highly sensitive anemometer has been 

 developed at the California Institute of Technology.^ 

 This instrument records wind speeds from 0.5 to per- 

 ha]3S 30 mi3h. It has the conventional three cups rotat- 

 ing on a vertical axis. Each rotation is registered on a 

 counter by magnetically operated electrical contacts. 

 For higher wind velocities the counter may be switched 

 to record only every hundred rotations. The apparatus 

 is delicate and is critical in its behavior toward certain 

 adjustments. 



'■^•* Semipermanent Installations 



TOWEIIS 



Two major installations of towers are at present in 

 existence in England. They are the Porton towers and 

 the Eye towers. The Porton towers on the Salisbury 

 Plain form part of the extensive meteorological equip- 

 ment of the British Chemical Warfare Service and 



have been in use for a consideral)le number of years.' 

 Continuous records of dry and wet bulb temperatures 

 at heights of i, 2.3, and 56 ft above the ground are 

 made. The elements used are platinum resistance 

 thermometers connected into bridge circuits and are 

 artificially aerated. The recording mechanism is lo- 

 cated near the bottom of the tower. 



A .similar set has recently been installed on the Eye 

 towers in Sussex which form part of a CH radar sys- 

 tem. Temperature and relative humidity are recorded 

 for heights of 4, 50, 155, and 360 ft above ground. The 

 resistance thermometers are similar to those at Porton, 

 but the moisture measurements are made with the 

 Gregory humidiometer described above. 



In a large research project on microwave refraction 

 carried on in the summer of 1944 by the Propagation 

 Group at the Eadiation Laboratory, a mast was erected 

 at one terminal of the path. Wet and dry bulb tempera- 

 tures are recorded continuously with the device de- 

 scribed in Section 7.1.4 at heights of 4, 16, 36, and 

 55 ft above the sea surface, these heights varying some- 

 what with the tide. The measuring elements are lo- 

 cated in one end of a horizontal piece of tubing 3 ft 



