270 



SCIENCE 



[N. S. Vol. XLV. No. 1159 



To secure an adequate suspension the top of 

 the cord was first passed through a snugly 

 fitting hole in a fixed wire draw-plate and 

 then attached to the shaft of a strong fixed hori- 

 zontal screw, above. On turning the screw the 

 bob could be raised or lowered at pleasure or 

 secured in any position in virtue of the fric- 

 tion of the screw. An old Kohlrausoh galva- 

 nometer with elliptic coils and a magnetized 

 steel mirror in a copper damper at its center 

 was found very serviceable. By placing the 

 astisizing magnet in difl^erent positions with 

 or against the earth's field, the periods could 

 be usefully varied from 1 second to over 6 

 seconds. 



Pendulums J to 1 meter in length were first 

 suspended from a single massive rigid stand- 

 ard; thereafter from a gallows between two 

 massive standards, carefully braced. In 

 neither case was I able to eliminate to develop- 

 ment of elliptic vibrations, however, resulting 

 either from the action of the induced cur- 

 rents on the magnetic bob (an effect to be 

 anticipated) or from vibrations at the sus- 

 pension. I did not therefore attempt to carry 

 out measurements, although from the rapid 

 motion, the sensitiveness was very marked, 

 ^ = .06° to .03° per x = l cm. being easily 

 available. A rotational effect should there- 

 fore be observable in 10 sec. The whole ex- 

 periment is an interesting one, regarded either 

 in its present bearing, or as an illustration of 

 a vibrating system of two degrees of freedom, 

 or of the laws of induction. 



9. The Bifilar Inductor Pendulum. — Though 

 not immediately connected with the present 

 subject, the following striking experiment 

 uses similar synchronized apparatus. A long 

 (1-2 meters) brass or copper rod or bob, B, 

 Fig. 8, is swung horizontally from two thin 

 vertical brass wires ww attached at the ends 

 of the rod and to the ceiling, or elsewhere. 

 These thin wires are the terminals of the 

 S3Tichronized galvanometer, (?, and the brass 

 rod swings parallel to itself, cutting the earth's 

 vertical magnetic field, H^,, normally. The 

 mean horizontal speed, 2/, of the rod may be 

 written in terms of the maximum speed, i/„ 

 (simple harmonic motion) as y = 'iyji' and 



if a is the amplitude of the pendulum, T its 

 period, Z its length, g the acceleration of grav- 

 ity, e is the mean electromotive force induced 

 and & the length of brass rod (bob), 



2a6ffr \g u 



In my pendulum 



= 20 em., fc = 100 cm., B.^=.A, Z = 400 cm., 



whence 



c = 8 X 10"' volts nearly. 



Thus it should be possible to measure e with a 

 moderately sensitive galvanometer, partic- 

 ularly so if its period is the same as that of 

 the pendulum. 



Incidentally one may observe that if a hori- 

 zontal wire 10 meters long is moved normally 

 through the earth's vertical field with a speed 

 of 2 kilometers per minute, as on a flying 

 machine, the difference of potential at the 

 ends would be over e = 10"- volts. The latter 

 would have to be measured electrostatically, 

 however, with an artificial earth like a large 

 insulated condenser. If this can be done, it 

 would suggest a method of registering the 

 speed of the machine. 



A number of experiments were made with 

 the above pendulum (r=^4 seconds) and the 

 synchronized Kohlrausch galvanometer, of 

 which Fig. 9 gives an example. The needle 

 of the galvanometer was not at rest, owing to 

 the proximity of trolley wires and the astasized 

 simple needle. Hence the fluctuations at the 

 two elongations. But apart from this, the 

 result is about x = 1 cm. between elongations 

 per meter of length of the bob of the bifilar 

 pendulum and a double amplitude of the latter 

 of about D = 40 cm. (screen at 4 meters). A 

 shorter pendulum, an astatic needle and an 

 external magnet strengthening the earth's 

 field at the galvanometer, would give smooth 

 results. D could be much increased, etc. It is 

 also obvious that a long rectangular coil 

 similar to the bifilar and on knife edges could 

 be used to multiply the effect of the single 

 bifilar circuit. 



Gael Barus 



Brown TJnivessity, 

 Pkovdjencb, E. I. 



