EQUIPMENT AT MOUNTAIN LAKES 



109 



Fic;ure 43. View of high-frequencj calibration Link, showing mechanical equipment Eor holding and positioning the 

 instruments. 



not be complete without emphasis on internally 

 doubly shielded transformers. These make it possible 

 to transmit or to receive, at will, on balanced or on 

 unbalanced lines. Under most circumstances, bal- 

 anced lines are much more effective in reducing cross 

 talk and the use of the internal shields on the trans- 

 formers results in the lumping of all stray capacities 

 between the shields, reducing cross talk still further. 

 These transformers also allow a better treatment of 

 the ground loop problem and are used for coupling 

 between all circuits and lines. 



Acoustical System 



The acoustical system, in general, consists of an 

 indoor calibration tank, mechanical equipment for 

 holding and positioning the instruments, absorbers 

 for reverberation control, transducers, and outdoor 

 test equipment. 



Calibrations may be made in the tank from 80 to 

 2,200 kc. The lower limit is determined by the rever- 



beration, while (he upper limit is imposed by the 

 response of the standard transducers. 



Calibration Tank of Positioning Equipment. The 

 calibration tank is made of %-inch steel and has an 

 elliptical cross section with approximate dimensions 

 of 7 feet for the major axis and 4 feet for the minor. 

 The depth of the tank is 4 feet. 



The tank has a capacity of 650 gallons and is 

 ecpiipped with a drain and a water inlet at the bot- 

 tom and an overflow pipe at the top. Water is pumped 

 directly from the lake, and as a result the problem of 

 fungus and slime assumes major proportions. Water 

 from a community system, as a rule, will contain the 

 same organisms, but in much smaller numbers. So 

 many different materials are used in the tank that 

 care must be exercised in the choice of a fungicide. 

 It must be noncorrosive, nonpoisonous, odorless, and 

 only slightly electrolytic. Furthermore, it must have 

 no effect on rubber. Experience showed that a 2 to 5 

 per cent solution of sodium dichromate was excellent 



