Page:Popular Science Monthly Volume 91.djvu/324

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��Popular Science Monthly

��effect can be obtained by lacquering the container black and polishing the nickel parts. — Thomas W. Benson.

��A Simple Arc Lamp Using a Thermostat Control

THE novel part of this arc lamp is the application of the active part of a thermostat for its control. A piece of

���One carbon-holder base is constructed of two metals to produce thermostatic action

asbestos board 3^ in- thick makes a good base. To this is attached a ring-bolt for hanging the lamp.

From copper tubing, y% in. outside diameter, cut two lengths each 2 in. long. A hole is drilled through the wall of each tube and threaded for a }/i-\n. bolt. One of these tubes is soldered to a strip of copper and clamped into position on the base by means of the binding post. The other tube is mounted on a heat-controlled strip formed by riveting together a piece of brass and a piece of sheet iron. The rivets should be spaced J^ in. apart.

Around this strip wind mica and then a single layer of No. 18-gage German silver wire. One end of this wire is led to the strip and the other to a binding-post. The carbons are slipped into the tubes and held there by screws. The two carbons are bent toward each other so that they touch. Care should be taken in mounting the com- pound strip so that the brass is next the asbestos base.

The operation will be clearly understood from the illustration. As soon as the cur- rent is switched into the arc it heats the German silver and causes the compound strip to bend, thus striking the arc. If the strip bends too far and breaks the arc add more wire and experiment in this manner until the arc burns steadily. It would be advisable to protect the strip from the heat of the arc by a shield made of asbestos board. — Thomas W. Benson.

��How to Make Slow Acting or Sluggish Relays

PRACTICALLY all electrical circuits, and especially telephone and telegraph circuits, require relays. In telephone work especially, where several relays are used in one circuit, it is necessary that some of them shall be slow acting or sluggish. Such relays are used in circuits for a variety of purposes, the most important ones being: First, to prevent temporary disturbances in one part of a circuit from affecting some other part. Second, to secure a certain time interval between the operation of different parts of a circuit.

A relay in itself is not generally slow operating or slow releasing but such features are determined by a combination of the design of the relay itself and of the circuit in which it is used.

Relays may be made sluggish in a num- ber of ways. The first is to equip the relay with either a heavy copper head at one end of the spool or a copper tube over the core the full length of the winding space. This is equivalent to a closed circuit winding having a single turn of very low resistance around the core. Similar re- sults could be accomplished with a regular short circuited winding, although this would not be as effective as the copper head or tube. It has been found that the copper head is more effective with tubular type relays, while the tube over the core is more effective with the return gravity armature type.

The use of either the copper head or tube results in any change in the field setting up an induced current in the short circuited winding in such a direction as to oppose the change in the field. Such an induced current will be of very low voltage as there is only one turn around the core, but of high amperage due to its low resistance. If the thickness of the copper head or sleeve is increased the resistance of this closed circuit will be correspondingly lower, and hence the induced current will be stronger for any given operating current.

The operation of every relay depends upon a certain magnetizing force known as ampere turns and is the product of the number of turns in the winding and the current passing through it. If the winding and operating ampere turns are so chosen that the relay receives just enough current to operate it, the field will not become strong enough to move the armature until the effect of the short circuited winding has

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