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up the light rays under test and is then connected to an amplifying circuit and then fed to the anode of the oscilliscope tube, which carries a fluorescent screen on one end where the electron stream in reality paints a picture of the wave form or of the vibration of the light rays.
In these examples as given, showing some of the representative uses where the needs of extreme accuracy could in no way be satisfied without the use of a photo-tube, a device which plays a part in attaining these results in no small degree, is the relay.
The use of the relay in various circuits plays no small part in the performance of industrial needs. The type of relay used in photo-tube control circuits is of necessity one which will respond to the minute flow of currents as had in the plate or output circuits of a small vacuum tube. A prime factor of relays used in these circuits is that of having an instantaneous response to the amount of current to which they are set. As the currents are very small, measured in micro-amperes, it is generally desirable to include in the relay circuit an extraneous impelling current working upon an additional solenoid to close the contacts carrying the current to the circuit which controls the operations of anything for which it is intended. This type of relay is known as the double action relay and is in general more sensitive than the single action relay which has only one solenoid dependent upon a few micro-amperes that the amplifying circuit is able to deliver to swing the armature. These extremely sensitive instruments are usually known as galvonometer relays and use currents in some cases as small as 0.000015 amperes which gives rise to the fact that the accuracy of the relay is deT rwe Iv e
pendent upon its range (micro-ampere rating) and on its torque (armature swing) . The approximate limits may be found in the following formula as given by “Weston”:
Examples —
High Value \of Adjustment Low Value \of Adjustment]
10.5 ma. 9.5 ma.
10
— O.lma. or 1% of mean value, (lOma)
72 volts 55 volts
10
—1.7 volts or 2.7% of mean value (63. 5v.)
This formula does not apply if the high value minus the low value is less than 10 per cent of the mean value (in this case 10. MA) . The above formula is given with the purpose in mind showing that the accuracy attainable depends upon the range of the instruments and is of no fixed value. It might in some cases be as close as +i4ofl% and in other cases
as wide as +10% of the mean.
However ,any standard micro-relay with its limits determined by the amplifier output in milliamperes may be used if the current rating of the contacts used to accentuate the controlling source, i.e., motor, heater, etc., is of sufficient capacity.
Bear in mind that the uses of the photo-tubes in the industrial field are by no means limited by the needs of the engineer. The next chapter on photo-tubes will go into the field of television, where the photo-tube takes the predominating part of the apparatus used in the obtaining of television images.
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A polygon with seven sides is called a hooligan.
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Queen Elizabeth was the “Virgin Queen.” As a queen she was a success.