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TRANS. ST. LOUIS ACAD. SCIENCE. 



a scale photographed on glass immediately below it. From this 

 needle an aluminum wire passes downward through the lens and 

 inclined mirror beneath to the other needle, which is below the 

 lantern. A small coil, surrounding the lower needle, serves to 

 deflect the system of needles, which is supported by a silk fiber. 

 The instrument may be made of any desired delicacy, and is 

 evidently a valuable addition to the apparatus of the public 

 lecturer. 



While meditating the construction of one of these instruments, 

 I hit upon another form of the instrument which appears to pos- 

 sess many advantages. A vertical section is shown in Fig. i. A 

 square box, Y Y, open at the top and bottom, is pierced on oppo- 

 site sides to admit the wooden rods, b. To the inner extremities 

 of these rods are attached coils, R, of covered copper wire, No. 

 i8, wound upon cylinders of wood, a. Wooden rods, c, clamped 

 so as to move with gentle friction, bear a wire, w, from which an 

 astatic system of needles is suspended by means of a fiber of silk. 

 The upper needle is midway between the centers of the two coils. 

 The lower needle plays over a scale, ^, photographed on glass 

 and graduated to fourths of a degree, beneath which is the hori- 

 zontal condensing lens of the vertical lantern. The needles are 

 ordinary sewing needles and are each 1.5 inches in length. Each 

 coil is composed of 34.7 meters of wire, the resistance of which 

 is 0.444 ohms. Each coil should have the same number of wind- 

 ings of wire, and the same resistance. This is easily effected by 



iFig. 23.] 



Scale !4., 



