Page 633 radio acoustic ranging 6732 



of Chicago, Illinois. All parts of this instrument are mounted on a heavy metal base. 

 On one end of the base is a. spool which holds the chronograph tape. Directly in front 

 of the spool are the magnets for operating the styluses; these magnets are mounted 

 on a hinged plate so that the stylus points may be raised while changing the tape. 

 The tape passes under the hinged plate and across a guide plate and thence between 

 two rollers by which it is drawn. The points of the two styluses contact the surface 

 of the tape where it passes over the guide plate. The rollers that draw the tape are 

 driven by a small electric motor which is coupled to the shaft of one of the rollers 

 through reduction gears. Most of these instruments are of the d-c type operated 

 from 12 volts, but a few contain 110-volt d-c motors. A lever is provided to separate 

 the two rollers so that the tape can be stopped without stopping the motor, and, to 

 facilitate the rethreading of the tape. The styluses may be adjusted as to pressure 

 on the tape surface, amplitude of motion, and position of the stylus point on the tape. 

 The speed of the chronograph tape is about 2 cm per second. 



The d-c resistance of the signal stylus magnet on most of these chronographs is 

 about 800 ohms, and that of the timing stylus magnet is usually about 5 ohms. On 

 some chronographs the magnets and styluses supplied originally have been replaced 

 with redesigned Baldwin piano units to which light styluses have been attached. Such 

 an alteration provides a quicker acting stylus and minimizes mechanical lag. Rochelle 

 salt crystals have also been used to actuate the styluses. 



A Gaertner chronograph of more recent design* is also used on some vessels of 

 the Coast and Geodetic Survey. Its principles are the same, the major differences 

 being in mechanical design and the use of a 110-volt 60-cycle synclironous motor to 

 move the chronograph tape. 



6732. Break-Circuit Chronometer 



A break-circuit chronometer is used with the Gaertner chronogTaph to actuate 

 the timing stylus once each second. It is lil^e any ship mean-time chronometer, except 

 that it is equipped with electric contacts operated by the chronometer movement. 

 These electric contacts are opened once each second for about one-tenth second. The 

 length of the break is adjustable, but it should not be attempted on board ship. The 

 operation of these contacts, when connected to the proper circuits, produces the 

 time offsets on the chronograph ' tape. These contacts are delicate and the current 

 through them and the voltage across them must be kept at low values. Some type of 

 relay circuit should be used between the chronometer and the cluonograph timing 

 stylus magnet; an electrically operated relay may be used, but a vacuum-tube circuit 

 such as is shown in figure 140 is preferable. 



6733. The Dorsey Chronograph 



The Dorsey chronograph was designed by the Coast and Geodetic Survey to pro- 

 vide more accurate timing of R.A.R. distances, and to be read without a glass scale 

 (4825). It uses the same tape as the Gaertner chronograph, but the speed of the tape 

 is two and a half times as fast, a time offset is marked each tenth second, and the 

 seconds are numbered. 



This instrument uses two styluses, one which marks tenths of seconds and one 

 which records the bomb and radio return signals. The speed of the tape is about 5 

 cm per second, so that the offsets marked at each tenth second are about 5 mm apart, 

 enabling the estimation of the time intervals to 0.01 second without using a glass scale. 



465382—44 42 



