264 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1954 



Company to use a magnetic ami)lifier to give continuous contiol of the out- 

 put of a co])per-oxide rectifier used for cathodic protection of undergi-ound 

 lead covered cables. The controlled rectifier was designed foi- use at a loca- 

 tion where straj' current "end effects" were damaging underground tele- 

 plione caliles and municipal light cables and were threatening high-pressure 

 water mains. When properlv adjusted, the output of the rectifier will in- 

 crease or decrease automatically so that at each instant the amount of 

 forced drainage will be adequate to protect the underground telephone cable 

 sheath but will not be in excess of the value at which neighboring under- 

 ground metallic structures would become anodic and would thus become 

 subject to corrosion. For satisfactory operation the amplifier had to be 

 designed to give a large gain so that a change in the control voltage of onlj' 

 0.2 volt (+0.1 to —0.1 volt) would be sufficient to vary the output of the 

 rectifier from practically zero current to full rating. In the installation de- 

 scribed in this paper the gain of the magnetic amplifier is in the order of 

 12,000. The magnetic amplifier type of control is well suited to outdoor in- 

 stallations subject to wide changes in temperature and at remote locations 

 where frequent maintenance inspections are not feasible. The magnetic am- 

 plifier has advantages over other available control devices which accomplish 

 this same purpose in that it has no moving parts, no vacuum tubes, bat- 

 teries, motors, relaj's or contactors. The magnetic amplifier gives a continuous 

 output control which is superior to the stepped increment changes that 

 result from relay or contactor operation. 



Jerone, M. G., see E. P. Smith. 



Johnson, J. B.^ and K. G. McKay^ 



Secondary Electron Emission of Crystalline MgO, Phys. Rev., 91, 

 pp. 582-587, Aug. 1, 1953 (Monograph 2163). 



Secondary emission is measured from single crystals of ^IgO cleaved along 

 the (100) plane. The maximum ratio of secondary to primary current, Smax , 

 is about 7 at about 1,000 volts and room temperatures. The cross-overs 

 are at 33 volts and far above 5,000 volts. Most probable energj- of emission 

 is 1 ev or less. A definite effect of temperature is established, decreasing with 

 increasing temperature, in accord with expectations for an insulator. 



Jones, T. A.^ and W. A. Phelps^ 



Level Compensator for Telephotograph Systems, Elec. Eng., 72, 

 pp. 787-791, Sept., 1953. 



To eliminate interference in telephotograph transmission through broad- 

 band carrier equipment, it was decided to cancel it from the signal delivered 

 by the carrier facilitj' instead of modif3-ing the carrier ecjuipment. Conse- 



1 Bell Telephone Laboratories. 



