673 HYDROGRAPHIC MANUAL PaGE 632 



tap of the line-to-grid transformer connects to the output of the radio receiver by means of separate 

 contacts on the above-mentioned switch. 



When the hydro-radio switch is in the position to receive the bomb signal, the 

 output of the radio receiver is short-circuited; and when the switch is in the position 

 to receive the radio signal, the output of the bomb amplifier is short-circuited. The 

 short-circuiting of one circuit while the other is in use is to prevent cross operation 

 between circuits. A double-pole double-throw switch that can be operated rapidly 

 can be used; but a preferable arrangement is a telephone-key switch connected to 

 operate for the reception of the bomb signal when held down, but which, when released, 

 will spring back to normal position and transfer the circuit to the output of the radio re- 

 ceiver. Whatever arrangement is used, the whole switching operation should require 

 no more than a half second, for the speed with which this switch can be operated deter- 

 mines the minimum time after the bomb explosion that a radio signal can be recorded 

 on the chronograph. The manually operated switch described above may be replaced 

 by a relay which automatically switches to the output circuit of the radio receiver as 

 soon as the bomb signal is received, as in the Dorsey chronograph (see 6733). 



The gain control for bomb reception is by regulation of the current flowing to the 

 carbon button of the hydrophone, a d-c milliammeter indicating the amount of current 

 flowing to this unit. Once the proper value of gain has been determined, readjustment 

 is rarely required. The gain for the radio signal is adjusted by the controls which are 

 part of the radio receiver. 



673. The Chronograph 



A chronograph is a graphic-recording time-measuring device, used in R.A.R. to 

 measure the time intervals between the bomb explosion and the reception of the radio 

 signals from the R.A.R. stations. The record is made on a narrow wax-coated paper 

 tape by two sharp-pointed electromagnetically operated styluses. The tape passes at 

 a constant rate imder the styluses which are adjusted so that their marks coincide to 

 mark a single line down the center of the tape when they are in an unactuated state. 

 (See fig. 141.) When actuated the two styluses move in opposite directions from 

 the center toward the edges of the tape, the maximum amplitude of motion being 

 about %6 inch. After operation, a stylus is returned to normal position by means 

 of a spring. The mark on the tape where the stylus leaves the centerline is used in 

 scaling all time measurements. One stylus marks the time and is known as the timing 

 stylus, and the other, operated by the chronograph amplifier, is known as the signal 

 stylus. The method of scaling the tapes from a Gaertner chronograph is described 

 in 6853. 



a. Chronograph tape. — The tape is a narrow red- or black-bodied paper, one surface 

 of which is covered with a thin white wax coating. The melting point of the wax is 

 sufficiently high to withstand tropic temperatures unless the tape is exposed directly 

 to the sun's rays. This tape comes in 300-foot rolls, wound on a wood center about 

 lYi inches in diameter with a )^-inch centerhole. The tape is about K inch wide and 

 0.002 inch thick. One roll of tape is sufficient for about 1 hour's continuous operation 

 on the Gaertner chronograph. 



6731. Gaertner Chronograph 



Most of the chronographs used in the past in R.A.R., and sometimes still used 

 by the Coast and Geodetic Survey, were made by the Gaertner Scientific Corporation 



