316 



Popular Science Monthly 



factories, when the starting rheostat 

 gets out of order. For a lOO h. p. motor, 

 a water rheostat of the dimensions 

 given below will be quite satisfactory. 

 Assuming that the voltage is no, so 

 that the amperes on full load amount 

 to 680, a fairly large-sized wooden cask 

 is necessary, to allow for a proper 

 dissemination of the heat which is 

 produced by the heavy current. Ac- 

 cordingly, a large wooden cask is filled 

 with slightly acidulated water — sulphur- 

 ic will do — and an iron plate having 

 four square feet of exposed surface 

 bolted to one side of the cask in the 

 solution. Another plate of the same 

 dimensions is lowered into the water 

 with a heavy, flexible, insulated cable. 

 When it is completely submerged, a 

 short circuiting switch is thrown in. 



Hairspring of a Watch as a 

 Detector-Point Support 



THE illustration shows a home- 

 made detector which may be help- 

 ful to those who do not need to construct 

 a more elaborate instrument. 



The hairspring of an old watch pro- 

 vides support for the point, and is an 

 effective shock absorber. It is held by a 

 drop of solder to the adjustment arm, 

 which is bent at right angles after insert- 

 ing it in its binding-post support. 



A one-inch length of annunciator wire 

 is sharpened to a point for the crystal 

 contact. Wind a few of the innermost 

 turns of the spring around it, as shown in 

 the drawing, and fasten with a well- 



A length of wire pointed and supported 

 by the hairspring for a crystal contact 



placed drop of solder to avoid loose 

 contact. 



If the conventional cup-type mineral 

 holder is not to be had, a good clamp 

 can be made very easily by bending up 

 one side of a small piece of sheet brass 

 as shown. Solder a nut taking an 8/32 

 in. screw directly opposite, and grip the 

 crystal by tightening the screw on it. 



Making a Miniature Light to Attach 

 to a Book 



/y/re inju/ated 'Jww 

 /amp jcreiTi 



M 



■Boo A coyer 



A tiny electric light for 

 attaching to book covers 



ANY a 

 sum- 

 mer evening 

 would be 

 more enjoy- 

 able if one 

 could sit on 

 the veranda 

 and read with 

 only light 

 enough to 

 illuminate 

 the book. 

 The same is 

 true in win- 

 ter, when there is an open fireplace. 



A simple book-light can be made by 

 bending a strip of brass, y% in. thick 

 and I in. wide, as shown in the diagram. 

 Bore a hole large enough to receive a 

 miniature lamp, which should fit snugly. 

 Attach two thin strips of brass (3^ in. 

 wide), to the other end of the first piece 

 of brass to act as a spring, which can 

 be slipped over the cover of the book. 

 The wire connections are shown in the 

 illustration. — Wm. E. Finkernagel. 



An Airtight Quenched 

 Spark-Gap 



WHETHER or not quenched spark- 

 gaps should be made completely 

 airtight has often been a subject of 

 dispute. The Telefunken gaps are 

 usually fitted with plain mica separating- 

 rings, and no great pains are taken to 

 exclude air from the space between the 

 sparking surfaces. 

 In spite of this, the 

 Telefunken trans- 

 mitters as a rule op- 

 erate efficiently and 

 with uniform, clear 

 spark-tones. The 

 effectiveness with 

 which a quenched- 

 gap prevents "back- 

 firing" depends to a [^ 

 large extent upon 

 the condition of the 

 parallel sparking- 

 surfaces. If these 

 are perfectly smooth and very slightly 

 oxidized, the operation of the apparatus 

 is usually most satisfactory. How long 

 the plates will remain in good condition 

 depends upon the material from which 



Construction of De 

 Forest's air-tight gap 



