Page:Handbook of Meteorology.djvu/253

 One of these wheels contains 100 teeth and the other 99. By means of this differential motion the registration of the number of miles up to 990 may be read directly.

A type of Robinson anemometer in which registration is made with index hands is also made. Its mechanism does not differ otherwise from the ordinary type. One of the train of geared wheels in the ordinary form of anemometer is cut with 50 teeth. If replaced with one cut with 62 teeth the disks will record kilometers instead of miles. If an interchangeable gear of this sort is desirable, it is better to have the necessary adjustments made by the manufacturer.

Several types of magnet recorders are used with the Robinson anemometer. In the type most commonly used, an arm carrying the recording pen is attached to the armature of the magnet. The revolving disk carries the studs representing miles of wind movement against the spring which acts as a circuit closer; the recording pen thereupon makes an offset from the straight line on the record sheet. Each offset represents theoretically a mile of wind. As soon as the stud passes the contact spring—a matter of from five to ten seconds—the pen is drawn back to its normal position. The fourth and fifth studs are bridged and the closed circuit makes the offset which covers a theoretical mile of wind movement. This is a convenience which enables the number of miles to be counted in groups of ten. Should any intermediate mile-stud fail to record, the failure will not be lost; it is included in the count.

Caution is necessary at times in reading the closed circuit mile, especially when it represents a high velocity whose measure is to be determined closely. The observer must choose between closing and closing, or between opening and opening of the circuit. Because the opening—the “breaking”—of the circuit is quick and positive, this interval is considered preferable in determinations.

The record sheet is necessary in finding the time of maximum or of minimum velocity of the wind. From it one may also find the velocity for any hour of the day. And inasmuch as such information is frequently required in suits at court, accurate dating and time-checking of the record sheets are essential.

The totals for any specific time may be read from the record sheets. Daily and monthly totals may be read from the dial.