Popular Science Monthly 



285 



it keeps the little mirror very busy trying to 

 follow them. 



As shown, the shaft of the moving part 

 consists of a small brass pin sharpened at 

 both ends and with bearings made by 

 small dents (not holes) in the two pieces of 

 thin sheet br<*ss. A small piece of iron 

 wire is fastened at right angles to the brass 



of the bearings on the shaft is made 

 adjustable so that the mirror will not 

 swing too far. This dampening effect is 

 quite desirable in making oscillograms of 

 rectified current, so that the mirror will not 

 swing beyond the zero point. With no 

 volt A.C. a lamp or other resistance should 

 be placed in series with the coil of wire. If 



MAM 



An oscillogram and a simple way of making the curve by drawing a piece of paper under a moving 

 pencil to illustrate the process by which the recording oscillations are made by the machine 



pin with a drop of glue or sealing wax. A 

 mirror (somewhat larger than the head of a 

 pin) is glued to the center of the iron wire. 

 The mirror is best made from a microscope 

 cover glass. This is a small square of very 

 thin glass and may be dropped into a test- 

 tube of silvering solution for which there 

 are various formulae. The test-tube should 

 be of such size that the glass will just fit 

 into it. This will support the glass upright 

 in the tube so that it will be evenly silvered 

 on both sides. The silver coating is after- 

 wards removed from one side of the glass 

 by touching it with the end of a tooth-pick 



a core is used, it should be made up of small 

 iron wires. The size and amount of wire 

 necessary on the spool or electromagnet is 

 best found by experiment. The spool is 

 fastened by rubber bands to the support, 

 making.it easy to substitute various spools 

 containing different sizes and lengths of 

 wire. Very little current is required as the 

 mirror is influenced by the electromagnet 

 at considerable distance from it. The- 

 mirror and moving part should be mounted 

 a little nearer to one pole of the permanent 

 magnet than the other, so that the spot of 

 light is adjustable by moving the permanent 



Oscillograms of a sixty- cycle alternating current before and after rectification, the zero line, 

 indicated by a straight line in the first two, being photographed without a current in the coil 



moistened with nitric acid. After washing 

 and drying, the mirror is broken into bits 

 and a small piece selected as nearly round 

 as possible. A piece of the silver chipped 

 off the back of an old mirror might be used 

 in place of a mirror. 



By means of the small screw, the tension 



magnet backwards or forwards as desired. 

 As a source of light, sunlight is excellent 

 for viewing the oscillograms directly. The 

 sunlight is allowed to fall on the mirror and 

 is then reflected on to a piece of white 

 paper as a spot of light. When the mirror 

 is being vibrated, the spot of reflected light 



