30 MANUAL OF CURRENT OBSERVATIONS 
Radio Current Meter* 
81. The radio current meter was developed by Capt. Elliott B. Roberts of the 
Coast and Geodetic Survey, and has been used for a number of years with satisfactory 
results. It is designed to measure both velocity and direction. It normally requires 
the use of considerable auxiliary equipment but has the very decided advantage that 
it can be operated from an anchored buoy thus obviating the necessity of maintaining 
a vessel and crew at each current station during the entire period of observations at the 
station. 
82. In the radio current meter a rotating impeller is actuated by the current. The 
impeller is connected through a magnetic drive to an enclosed interior mechanism which 
makes and breaks an electric circuit by means of two contacting devices. One makes- 
a contact at each fifth turn of the impeller, the frequency of these contacts serving as 
a measure of current velocity; the other makes a contact at each tenth turn of the 
impeller. The first contacting device is connected with a built-in magnetic compass; 
the second is fixed relative to the meter body. The devices are so arranged that when 
the instrument is heading south both contacts will occur at the same time. When the 
meter heads in any other direction, the time relation between the two sets of contacts 
changes with the meter heading. This time relation, therefore, serves as a measure 
of the direction of the current. 
83. In operation, the meter is suspended from an anchored buoy. The buoy 
houses a radio transmitter, complete with batteries, which is connected electrically 
with the meter and with an antenna mounted on the buoy. The contacts made within 
the meter are thus relayed as radio signals by the transmitter to a receiving station. 
84. The receiving station may be either aboard ship or on land. It is equipped 
with a suitable radio receiver, a chronometer which furnishes the time reference for the 
signals, and a chronograph for recording the time in seconds and the radio signals. 
85. From the chronograph tape the number of seconds between velocity con- 
tacts is determined and the corresponding velocity is taken from a rating table 
prepared from a calibration of the meter. The direction is read directly from 
the tape by means of a converging scale having graduations from 0 to 360 degrees. 
The zero- and 360-degree graduations are placed upon adjacent velocity marks and the 
direction read on the direction mark. Direction marks are distinguishable by the 
double time interval. There is a direction mark between every other pair of velocity 
marks. 
83. The buoy used with the radio current meter is specially designed for that 
purpose. It is described in detail in the radio current meter operating manual. The 
problem of anchoring the buoy has been given considerable study some of the results 
of which are shown graphically in Figures 22 and 23. Figure 22 shows the length of 
7x 7x % (%« inch in diameter having 7 strands of 7 wires each) stainless steel cable 
required for anchoring the 80-inch buoy in different depths and current velocities. The 
limiting condition shown by the dashed curve is the greatest anchoring depth possible 
in the given current velocity. Figure 23 shows similar data for a 120-inch buoy using 
7x 7x \% stainless steel cable. The procedures followed in planting and recovering the 
buoys are discussed in paragraphs 105-109. 
*The original model of this instrument has been employed in a number of current surveys. The methods of use are described in 
detail in Roberts Radio Current Meter Operating Manual. An improved Model II, now available, is described in a revised (1950) 
edition of the Manual. 
