308 



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 



