208 



NA TURE 



|_ June 26, i'x 



/< 



r tt + E) 



where/ is the greatest stress in the material, i the thickness of 

 the strip, N the modulus of rigidity, and E Young's modulus for 

 the material. 



The authors now show how their springs may be used to 

 determine directly the ratio of the modulus of rigidity of a 

 material to its Young's modulus, and they conclude their paper 

 by describing some practical applications of their springs which 

 they have already made in measuring-instruments. Thus by 

 the employment of springs such as those described, they have 

 succeeded in making ammeters and voltmeters, or instruments for 

 measuring respectively electric currents and differences of poten- 

 tial, in which the pointer moves over in some cases as much as 

 270° of the scale instead of only 50°, which is all that can be 

 obtained with ordinary galvanometers. One form of the instru- 

 ment is shown in Fig. 3, where A A is a thin, hollow tube of 

 charcoal iron attached at its lower end to a brass piece G guided 

 at the bottom in the way shown. To G is attached the lower 

 end of a spring made in the way described, of silver or hard 

 phosphor-bronze, the upper end of which is attached rigidly by 

 a thin rod to the glass top of the instrument, which itself is 

 fastened rigidly to the framework of the instrument. The rod 

 attached to the glass, and to which the upper end of the spring 

 is attached, also serves as a guide to the top of the iron tube. 

 In the space F F a solenoid wire or strip is wound, its ends being 

 attached to the terminals shown. Hence, when a current is 

 passed through the wire, the iron tube is sucked into the solenoid, 

 and its lower end G, to which the spring is attached, receives ,1 

 large rotatory motion, which is communicated directly to the 

 pointer attached to the top of the iron tube. Parallax in taking 

 readings of the pointer is avoided by the horizontal scale being 

 on looking-glass in the well-known way. 



By making the iron tube A A very thin, so that it is mag- 

 netically saturated for a comparatively weak current, by fixing it 

 so that it projects into the solenoid a fixed distance which has 

 been carefully determined by experiment, and by constructing 

 'he spring in conformity with the conditions worked out in this 

 paper, so as to obtain a large rotation with minimum stress, and 

 with not too much axial motion of the free end of the spring, 

 1 hey have succeeded in obtaining deflections up to 270 directly 

 proportional to tlte current, and without any permanent set 

 heing given to the spring. 



To prevent a spring taking a permanent set for a large deflec- 

 tion, it is of great importance that the spring after being de- 

 livered by the maker should receive a large degree of permanent 

 set in the direction in which we wish it to be afterwards strained 

 i 1 ordinary working. 



In spite of the fact that Prof. T. Thomson in the Cambridge 

 mid Diiblin Math, fount., November 1848, explained the 

 importance of initial strains in materials, the reason is not yet 

 sufficiently well understood why when a round bar has been well 

 twisted beyond the limit of permanent set in a certain direction 

 it lias twice as much elastic strength to resist torsion in this 

 direction as in the opposite direction. Now in the very act of 

 manufacturing these springs, that is in the bending of the strip, 

 the material acquire- strains which are just opposite in character 

 to the initial strains which we wish it to possess, for, as already 

 explained, if the spring be constructed as in Fig. 1, an extension 

 of the spring produces a rotation tending to uncoil it. Hence a 

 spring must not be regarded as ready for use until it receives a 

 good set by means of a weight hung from its end. 



This instrument is direct reading, the adjustment for sensibility 

 being made by a email slvJing coil th: j ill- petition of which 

 is initially determined experimentally by the makers, ami in 

 which position the coil is permanently fixed. 



Theory 'of the Solenoid Spring Ammeter orVoltmeter.— If C is the 

 .111 rent in amperes flowing through the coil, the attractive force 

 on the iron core is 



KC g 

 I + SC 

 where S is a constant, which is the greater as the current is 

 smaller for which the iron tube A A, Fig. 3, becomes saturated 

 with magnetism. The position ,,1 this iron core in the solenoid 

 is so selected that K remains practically constant throughout the 

 small range of downward motion of the core. 



Since the rotation tp has been produced by an axial force, we 



know hum the theory of the spring already given, that this axial 

 force is/0, where p is some constant. Hence 



K C 3 



1 + sc 



P<r 



and since S C is great in comparison with unity for such 

 as we wish to measure, we have 



,, S t> 



s 



that is, equal divisions of the scale corresporftl with equal addi 

 tions to the strength of the current except close to the zero, and 

 the authors do not usually graduate these instruments within t 

 of the zero. 



Shielded Measuring-Instruments. — When it is desired to use 

 the instrument close to a dynamo machine or electromotor in 

 action, they have adopted a different and somewhat special form 

 of construction, which is shown in Fig. 4, by means of which 

 the instrument is to a great extent shielded from even powerful 

 external magnetic fields. In this instrument the electro- 

 magnet consists of a hollow core, part of which, B B, is of char- 

 coal iron, and part, n 1-;, of brass, or other non-magnetic metal. 

 The outside tube, C C, and the plates, X X, top and bottom, 

 are also of charcoal iron. The space F F is filled with insulated 

 wire or strip in electric connection with the terminal, so that when 

 a current is sent through the instrument an intense magnetic field 

 is funned between D and E, which are the poles of the electro 

 magnet. To the iron tube A A, also made of charcoal iron, the 

 spiral spring, in this case made of extremely thin hard steel, is 

 attached, the other end being attached to the piece F, which is 

 fixed relatively to the bobbin. The spindle G G, which is fixed 

 to the moving iron ce>re A A, moves freely in bearings H H, so 

 that the only movements of which A is capable are one of rota- 

 tion and one parallel to the axis of the bobbin. As the iron core 

 A projects into the strong magnetic field between D and E, 

 it is strongly attracted towards E when the current flows, and. 

 as before, causes a large rotation of the pointer p over the 

 scale. As a means of varying the power of the instrument 

 an adjustable iron piece K is provided, which can be screwed 

 nearer to or farther from the core A. and by the use of which 

 the sensibility of the instrument can be adjusted so as to make 

 the instrument '"direct reading," that is to say, each division of 

 the stale can he made to correspond with I ampere of current, 

 or 1 volt difference of potential, and the employment of a con- 

 stant such as 1 -34 amperes, or volts, per degree, which has 

 hitherto been necessary with our measuring-instruments, is now- 

 avoided. 'Ibis power of adjustment produced by the use of 

 the movable iron piece K, combined with the ease with which 

 more or less wire can lie wound on to the instrument, which also 

 constitutes a second adjustment of sensibility, is of cons 

 importance, since the employment of a constant has not only led 

 to error and delay in measurements made in electric-light fac- 

 tories, but has caused the breakage of the pointer or the destruc- 

 tion of an instrument from a far too powerful current being sent 

 through it by an observer (often a man with little experience in 

 the employment of instruments) having confounded the constant 

 of some other instrument with that of the one he was using. 



In the first of these magnifying spring ammeters and voltmeters 

 made by the authors, the instrument did not show the direction 

 of the current, but they have since added on the base of the in- 

 strument a small compass needle (not seen in the accompanying 

 illustrations), which points out at which of the terminals the 

 positive current enters, while the main pointer of the instrument 

 -laws as before the magnitude of the thing to be measured. 



Weighing-Machine'. — Another class of instruments in which 

 they have practically employed this spring are weighing- 

 machines, and Fig. 5 shows one of the arrangements adopted. 

 The scale-pan is prevented from turning by the part A B being 

 square and fitting very loosely a square hole in C. This arrange- 

 ment introduces practically no friction, and prevents the moment 

 of inertia of the scale-pan and load interfering, by means of a 

 rotatory motion, with the rapidity with which the pointer comes 

 to rest when a load is put into the pan. The position of the 

 pointer p, which revolves when a weight is placed in the scale- 

 pan, is read off upon the spiral scale D, which in the specimen 

 shown was graduated in pounds. In another of these weighing- 

 machines, shown in Fig. 6, the arrangement is the same with 

 the exception that a cylindric scale D is fixed to the end of the 



