41 



82. Magnifying and recording mechanisms. — In the barograph 

 illustrated, the barometric changes are magnified five times and re- 

 corded on a vertical drum adapted to embrace a change of 2 inches of 

 pressure and revolving once in three days, moving at the rate of 

 nearly a quarter of an inch per hour. A long experience with a 

 variety of scales indicates that records on time and pressure scales 

 of about the above proportions give, on the whole, the most satis- 

 factory and graphic picture of ordinary barometric oscillations. 

 Even the sudden changes that sometimes occur with thunderstorms 

 are very well brought out ; but for the most detailed effects of this 

 character a more rapid time scale is necessary. The magnification 

 is sufficiently great to show clearly the small fluctuations of from 

 a few thousandths to some hundredths of an inch that sometimes 

 occur for hours at a time. 



In the siphon form of barometer the change of level of the mer- 

 cury in either leg is only half of the whole change, assuming both 

 legs to have the same diameter, and, since we measure effects in the 

 open leg only and desire a fivefold magnification, it follows that an 

 actual tenfold magnification of the movements of the float is nec- 

 essary. This is accomplished by a large and a small wheel operating 

 on the principle of the wheel and axle, as may be clearly seen in 

 Figures 25 and 27. This construction provides a perfectly balanced 

 system which is itself neutral in all positions and, at the same time, 

 admits of a wide range of movement, results impossible to secure 

 with lever systems commonly employed in cases of this kind. 



In order to secure the strength of construction and at the same 

 time reduce friction to a minimum, the multiplying wheel and axle 

 are mounted on carefully designed and constructed ball bearings, 

 each cell containing only six balls, each one-sixteenth inch in diam- 

 eter. The ends of the axis entering the ball cups are 70° cones, 



A conical steel float, with the base somewhat hollowed out so as 

 to conform fairly well with the shape of the surface of mercury rests 

 lightly upon the top of the column and is suspended from the small 

 drum of the wheel-and-axle system by means of a narrow platinum 

 ribbon about 0.001 inch thick. The pen carrier is suspended by a 

 very fine copper wire running in a groove in the rim of the large 

 wheel, the diameter of which is approximately 5 inches, while that 

 of the drum is one-tenth as great. The exact ratio of these wheels 

 is made so as to realize a fivefold magnification of pressure changes ; 

 due account being taken of any slight differences in the diameters 

 of the open and closed chambers of the barometric column. 



To realize a condition of minimum friction great attention is neces- 

 sary in the design and arrangement of the pen carrier. First, the 

 weight must be the least practicable, since the mass of the float must 

 be somewhat in excess of ten times that of the pen carrier, and any 

 unnecessary weight in these parts introduces avoidable pressure and 

 friction on the axle. Second, while the pen carrier is guided and 

 constrained to move without sensible looseness in a definite vertical 

 line by sliding along a fine, stretched wire, nevertheless the whole 

 arrangement is so poised and balanced that if not disturbed by exte- 

 rior influences the carrier will rise and fall in exactly the same ver- 

 tical line, as nearly as may be, even when the wire is removed. This 

 adjustment serves to eliminate any sliding friction experienced bv 



