KN()\\Li:i)r-,i. 



Al-Kll.. 1912. 



till- hidck is free to move horizontally to .ind from 

 till- m,ij,Mut. (See FiKiiri- l.H.) The toji of this 

 woojiii l)l()ck is covered with velvet and supports a 

 liKlit lever with fjlass pen, as shown in the <iiaf,'rain. 

 The lever is counterpoised, and is centrallv balanced 

 liv two needle-points that rest on the velvet. The 

 pen is a glass tube drawn to a point in a Hame and 

 sealed, the sealed end being then ground down 

 carefully on a hone until the aperture is just 

 reached. It is, in fact, the same sort of pen as 

 is usid tor the harmonograph, and almost any 

 ordinary limpid ink can be used, preferably with 

 a little gum to prevent too free a flow. The 

 pen-point is allowed to rest 

 on a clock - face so as to 

 trace a circle upon it once 

 in twelve hours. At ever\ 

 discharge of the electricity in 

 the relay the electro-magnet 

 is brought into play and 

 the pen-lever on its wooden 

 support is quickly drawn aside, 

 so that a line across the circle is traced 

 number and proximity of the cross-lines will, tliere- 

 fore, be proportionate to the amount of electrification 

 of the air. and the exact time of each discharge will 

 be shown by the hour-lines on the dial-face. 



If any difficulty is experienced in making the glass 

 pens the tracing can be done on smoked paper with 

 a needle, but the ink records are obviously more con- 

 venient and more easily preserved. 



Suppose the apparatus to be set to discharge when- 

 ever the potential difference rises to, sav, two hundred 

 and ten volts. This can be arranged for by a little 

 trial with an electric light 

 main of that voltage, 

 the distance between 

 the copper foil and 

 the metal plate below 

 it being adjusted until 

 the two hundred and 

 ten volts just suffices to 

 attract the upper plate 

 dow n to the lower. This 

 having been arranged, 

 it follows that the 

 record of cross lines on 

 the dial will mean that 

 every cross line corres- 

 ])onds to the moment at 

 which the potential difference of the atmosphere was 

 sufficient to charge the instrument up to two hundred 

 and ten volts when it automatically discharged or 

 overflowed. The comparative frequency of the lines 

 in a given interval of time indicates, therefore, the 

 electric condition of the atmosphere. 



The adjustment of the distance between metal 

 plate and cojjper foil may be con venient 1\- 

 made by laying thin lead or brass strijjs on the 

 lower plate until the required level is reached, 

 and the final adjustment mav be effected b\- 

 tilting the relav verv sliglulv i)v means of a thin 



-f- 



tl 



Figure 134. 

 Th 



FiGUU)-; 135. 



wedge inserted unrUr f)ne or the other end. 

 Unfortunately, the record gives no intimation of 

 the sign of the atmospheric electricitv, a somewhat 

 important point from a meteorological aspect, but 

 as a record of mere electric disturbance the 

 apparatus is reliable and interesting, and is verv 

 easily constructed. 



.\ specimen of a ty[)ical record under normal con- 

 ditions of the atmosphere is reproduced herewith. 

 (See Figure 135.) Should the electricity be so 

 intense as to make the lines repeat themselves 

 with such frequency that they run into each 

 other, a small Leyden jar may be attached to the 



^^___^^_ collector so as to increase its 



rapacity, and consequently ex- 

 tend the time taken to bring it 

 up to the critical potential 

 difference. 



One of the first difficulties 

 with this recorder is the 

 tendency of the relay needle 

 to adhere to the mercury 

 instead of springing back to its position of 

 equipoise, and the result may be that the battery 

 will run down. But this gencrailv results from 

 sc^tting the instrument too close, so that after the 

 discharge it touches the mercury too lightly to 

 withdraw automatically. .\n effective way of pre- 

 \enting the adherence is to interpose a small 

 trembler bell in the circuit, the bell resting on the 

 top of the relay. It will, of course, ring every time 

 a record is made, and its vibrations, communicated 

 to the needle in the mercurv, will act like a 

 decoherer. The bell also has the advantage of 

 adding slightly to the 

 resistance of the circuit, 

 and thus of somewhat 

 lessening the strain on 

 the battery. Two dry 

 cells are ample, and one 

 will generally suffice. 



Perhaps the chief diffi- 

 cult v in the whole 

 science and art of col- 

 lecting atmospheric 

 electricity is that of 

 preventing spider lines 

 from earthing the out- 

 door part of the collector 

 itself, especially at certain 

 seasons of the year. .\ single gossamer is enough 

 to stop all signs of electricitv, and no matter how 

 high the collecting pole mav be it is never out of the 

 reach of the spider, which has a habit of making its 

 web at night, when the automatic recorder should 

 he of the greatest value. Even a coat of bird lime 

 on the pole does not stop the spiders. With a 

 radio-active collector made in windmill form, the 

 difTicult\- is almost entirely surmounted as the 

 web is broken by the revolution. With the water 

 dropping apparatus, there seems to be no satis- 

 factory way of guarding against spiders' lines. 



