This paper covers the initial attempt on the part of the Ohio Bell Telephone Company to use a magnetic amplifier to give continuous control of the output of a copper-oxide rectifier used for cathodic protection of underground lead covered cables.
The controlled rectifier was designed for use at a location where stray current “end effects” were damaging underground telephone cables and municipal light cables and were threatening high-pressure water mains.
When properly adjusted, the output of the rectifier will increase 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 underground 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 only .2 volt (+.1 to —.1 volt) would be sufficient to vary the output of the rectifier from practically zero current to full rating. In the installation described 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 installations subject to wide changes in temperature and at remote locations where frequent maintenance inspections are not feasible.
The magnetic amplifier has advantages over other available control devices which accomplish this same purpose in that it has no moving parts, no vacuum tubes, batteries, motors, relays 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.
