Popular Science Monthly 



been overcome and the field reaches its 

 maximum strength. Such a relay will be 

 slow in operating but may be~ made quick in 

 releasing by choosing such a design that the 

 releasing requirement will be high compared 

 with the operating requirement. 



If the winding and operating require- 

 ments are so chosen that the final value of 

 the magnetizing force which the relay 

 receives is much greater than the releasing 

 requirement, then the relay will be slow 

 releasing, as the magnetizing force will not 

 decrease enough to allow the release of the 

 armature, when the circuit is opened, until 

 the effect of the short circuited winding has 

 been overcome. Such a relay may be made 

 quick in operating by choosing such a 

 design that the releasing requirement will 

 be low compared to the operating require- 

 ment. 



In neither of the above cases is the 

 rapidity of the movement of the armature 

 itself greatly lessened, the greater delay 

 occurring between the time of closing or 

 opening the circuit and the beginning of the 

 armature movement. The above con- 

 struction is sometimes used for making a 

 relay that will not readily respond to 

 alternating current. 



A second method is to use an external 

 inductance or a non-inductive resistance 

 which come under the classification of 

 circuit design rather than the design of the 

 relay itself, except where the non-inductive 

 shunt is wound on the relay merely as a 

 matter of convenience. Either of these 

 means is used to cause the current through 

 the winding to rise or fall more slowly than 

 it would if no outside means was used to 

 affect this time interval. The non-induc- 

 tive shunt slows down the time of release 

 but has practically no effect upon the time 

 of operation, while the external inductance 

 slows down the time of operation but has 

 practically no effect on the time of release. 

 Both effects may be accomplished by the 

 use of the two in combination. As in the 

 case of the first method the movement of 

 the armature itself is not actually retarded. 

 The action of such an arrangement is to 

 increase the time between the closing and 

 opening of the circuit and the beginning of 

 the armature movement. 



A third method is to make the moving 

 parts of the relay heavy so that it will be 

 slow in responding to changes in the 

 magnetizing force. If the operating cur- 

 rent is just great enough to pull up the 

 armature, the relay will be slow in operat- 



309 



ing. To make such a relay slow in releas- 

 ing, the restoring force, whether gravity or 

 a spring, must be as small as possible and 

 still cause the armature to fall back. 

 Contrary to methods I and 2, with this 

 construction the actual movement of the 

 armature is retarded. Such relays are 

 used extensively on alternating current, as 

 their heavy moving parts prevent the 

 opening of the relay contacts during the 

 reversals of the current. 



The circuit conditions in each case 

 determine which of the above methods 

 should be applied, although the first and 

 second methods are the ones most com- 

 monly used. In some cases two of these 

 methods are used on the same relay to 

 meet certain peculiar circuit con- 

 ditions. — F. H. Tillotson. 



Reversing Rheostat for Controlling 

 a Small Motor 



IT is often desired to reverse the direction 

 of rotation of direct current motors and 

 at the same time adjust the speed to suit 

 the new condition of operation. A service- 

 able controller may be made as shown in 

 the sketch. The sets of contacts A, B, and 

 C, should be of brass or copper, and mount- 

 ed on a slate slab 12 in. square. The re- 

 sistance coils R should be fixed to the back 

 of the slate board. These coils, made of 



LINE 6RUSHES FIELD 



A reversing switch in connection with a 

 rheostat for controlling a small motor 



German silver, should have sufficient re- 

 sistance to give the proper speed control 

 without over-heating. 



Along the edge of the slate six binding 

 posts are arranged and connections made, 

 according to the diagram, on the reverse 

 side of the panel. The switch arm D 

 may be made from wood. The brass spring 

 contacts E and F are connected so that 



