and quickly discharges the capacitor (Cj) through the relay coil (Kj). 

 The 'surge' of current passes from K^ to a solenoid in the LWI pen 

 arm relay, causing the relay to close. When the relay closes, the pen 

 arm is forced downward. The pen arm returns immediately to its 

 original position, since the increased electric current is only momen- 

 tary. With Ci now discharged, the transistor (due to its inherent 

 nature) reverts to its original non-conductive state and the capacitor 

 starts charging again. This procedure repeats over a 7 to 10 minute 

 period as long as the electric circuitry to the 12 V battery is con- 

 nected, i.e. as long as the sensing element is causing the LWI sensor 

 relay to remain in the 'wet' position. 



Instrument Shelter 



A shelter is necessary to house the hygrothermograph, rain 

 gauge digital counter, and other items. The model pictured (see 

 cover) is the U.S.A. Weather Bureau Instrument Shelter, but other 

 suitable commercial shelters are available, and home-made shelters 

 with similar features are also acceptable and will likely be much less 

 expensive. The shelter should be white, and should have louvered 

 sides or other allowances for air exchange to insure accurate 

 ambient temperature and relative humidity readings. 



CONSTRUCTING THE 4-PEN ARM DESIGN 



Preparing the LWI and Rain-intensity Pen Arm Relays 



Materials 



(1) 12 VDC relay, KRPl IDG (AMF/Potter & Brumfield, Prince- 

 ton, Indiana 47671, U.S.A.) 

 (1) 6 V 50-60 HZ relay, KRPl 1 AG (AMF/Potter & Brumfield) 



Instructions (refer to Fig. 7) 



1. Remove plastic hex nut and four screws from plastic relay 

 housing. Discard screws and plastic case. 



2. Cut the eight wires leading to contacts and coil (a) 



3. Remove and discard the fibreboard mount (b) 



4. Temporarily remove retaining spring on back of relay. 

 Remove metal plate with attached copper contact strips. 

 Bore through the two rivets securing the contact strips to 

 plate. Discard the contact strips. 



5. Replace metal plate and retaining spring (c) 



