477 



HYDROGRAPHIC MANUAL 



Page 416 



477. Roberts Radio Current Meter 



A radio current meter, designed by an oj0&cer of the Bureau, was tried out experi- 

 mentally in 1940, and in 1942 in a current survey in Puget Sound 12 stations were 

 occupied using 3 of these instruments. At one station a series of 20 days' observa- 

 tions was secured, with the longest break of 8 hours' duration. The breaks in the record 

 were due principally to radio interference and static. 



The principle of the instrument is simple: A powerful magnet, pivoted so it is free 

 to aline itself in the magnetic meridian, is mounted in a streamlined casing with fins 

 which cause it to stream with the current at all times. By revolution about an axis 

 concentric with the axis of rotation of the magnet, an electric contact is made succes- 

 sively with one point attached to the magnet and one point on the casing. It is apparent 

 that the direction of a current will be indicated by the radial relation of the two contacts 

 in one revolution cycle. 



If the electric contact is revolved by the force of the current, the time of revolution 

 is a measure of the velocity of the current. 



The meter is suspended from a streamlined buoy as described in 2843. Radio 

 signals are transmitted, when the electric contacts close, by a transmitter similar to 

 that used in Radio Acoustic Ranging (see 6433, 6513, and 6523) which is installed in the 

 buoy. The radio signals are recorded on a chronograph tape, such as is used in R.A.R. 

 (see 673 and 6853), which is installed ashore or on boarc^ the vessel used to service 

 the buoys. 



Figure 90 is a simplified sketch illustrating the operating principle of the meter. The oil-filled, streamlined casing ylhas fins to 

 cause it to stream with the current, when suspended freely, and an impeller B revolved by the current. The latter transmits its motion 

 to the contacting mechanism in the sealed interior of the casing by a magnetic coupling C, so designed as not to affect the magnet. 

 The sketch does not show the forward part of the screw impeller, nor the afterbody of the shell, which contains an expansion chamber 

 to equalize pressures inside and outside. 



SCALE IN INCHES 



Figure 90. — Radio current meter. 



A compass magnet D, movmted in gimbals E and damped by the oil in the casing, bears a spring-mounted point F. Another spring- 

 mounted point G is fixed to the frame. By shafts and gears from the magnetic coupling a planetary gear system is revolved at a speed 

 proportional to the velocity of the current. The rotating base gear H has mounted on it the planetary gear J, which is geared into the 

 fixed pinion /. The relationship of the gears is such that the planetary gear J rotates about its axis once while it is making two revo- 

 lutions about the axis of rotation of the magnet. 



The upper part K of the planetary gear J makes the contacts with the points F and G as it revolves past them. Since K is rotating 

 slowly about its axis, its contact with i*" is a rolling contact, which exerts almost no tangential force. This results in little or no effect 

 on the position of the compass magnets, even though a firm contact pressure may be exerted. 



The compass magnet and its contact Fare always alined with the magnetic meridian,and the casing and its contact G are oriented 

 with the direction of the current, so the two series of signals have a phase relationship from which the direction of the current can be 

 deduced when they are recorded on a chronograph tape. The two series of signals are identified from the fact that the signals from 

 contact F are twice as frequent as those from contact G, since contact with F is made every revolution but contact with G is prevented 

 every alternate revolution by a cut-out segment on one side of K. 



