152 



SCIENCE. 



[Vol. YL, No. 133. 



dispositions were made to render them inoperative. 

 The chief recommendation of this method is, that its 

 mechanical execution is so extremely simple as to be 

 within the reach of even an unskilled mechanician. 

 Of course this contrivance entirely fails to meet the 

 requisite of portability. 



If the conducting circuit is bent into a circle, and 

 the needle placed at its centre, we have the tangent 

 galvanometer, which may be regarded as the funda- 

 mental type among electrical measuring instruments. 

 As in this case, the tangent of the angle of deflection 

 is proportional to the current strength; with a great 

 increase in the magnitude of the latter, the angle 

 becomes too large for accurate measurement, owing 

 to the rapid variation of the tangent for large angles. 

 This difficulty can be obviated by increasing the ra- 

 dius of the circle; but the instrument then becomes 

 gigantic and unwieldy, and is no longer portable. 

 Increasing the strength of the magnetic field is an- 

 other remedy; and another still is found in increasing 

 the distance of the needle from the circle upon a 

 scale calculated from the theory of the instrument, 

 and verified or corrected experimentally. 



The last two devices are employed in Thomson's 

 current galvanometer and potential galvanometer. 

 In these, the coil is made of comparatively small di- 

 mensions, as is the whole instrument, rendering it 

 very portable and convenient to use. A semicircu- 

 lar magnet placed over the coil gives a very intense 

 magnetic field, diminishing the deflection of the nee- 

 dle to a suitable value when powerful currents are 

 used; while the compass-box can be moved away 

 from the coil along a graduated scale. These ar- 

 rangements give the instruments great range in their 

 indications; but as the intensity of the curved mag- 

 net is easily affected, they require constant verifica- 

 tion. In a laboratory this is very easy, and the 

 apparatus is admirably suited for a variety of appli- 

 cations; but its delicacy, and the need of constant 

 oversight, render it unsuited for rougher work. 



Various other methods may be resorted to for in- 

 creasing the range of the tangent galvanometer. 

 Shunting is a practical and useful device, if care is 

 taken that the shunt is so placed, or so far removed, 

 as not by itself to deflect the needle, or affect the 

 intensity of its field. The current may be passed 

 through another parallel circuit of smaller radius, so 

 that only a differential effect is produced upon the 

 needle by the two portions of the circuit, as has been 

 done by Brackett. Or the coil may be made to turn 

 about a horizontal axis, as in the cosine galvanometer 

 of Trowbridge, re- invented later by Obach. 



As in all these instruments, the effect upon the 

 needle is dependent upon the intensity of the mag- 

 netic field; and this is usually that of the horizontal 

 component of the terrestrial magnetism: the varia- 

 tion in this may occa>ion considerable errors unless 

 its value is constantly and accurately known; and 

 there is also the liability to the intrusion of foreign 

 magnetic forces from the circuits and magnetic 

 masses in the neighborhood, a cause of error which 

 is by no means imaginary in practical cases. 



A second class of instruments dispenses with the 



needle, and utilizes the action of a fixed circuit upon 

 a movable one, which is traversed either by the main 

 current, or a shunted portion of it, or an independent 

 current which can be varied or controlled. One of 

 the simplest forms consists of two circular parallel 

 circuits, either single, or consisting of many turns, 

 one of these circuits being freely movable. The 

 strength of the current is then directly proportional 

 to the force required to keep the circuits at a fixed 

 distance apart. This has the important advantages 

 of entirely avoiding the use of magnets, and of equal 

 applicability to steady, variable, or alternating cur- 

 rents. While disturbing magnetic effects are not en- 

 tirely excluded, they are not usually of serious import. 

 No practical and compendious apparatus embody- 

 ing this principle is in general use, though it has 

 been employed with success in Hill's dynamometer. 

 Further experiment in this direction seems desirable. 

 The various forms of dynamometer in use generally 

 have the movable circuit mounted, so as to turn about 

 a vertical axis, like the needle of a galvanometer, the 

 strength of the current being computed from the 

 angle of deflection, or read from an empirical grad- 

 uation. In Siemens's form, however, the movable coil 

 is brought into a fixed relation to the stationary one, 

 by torsion, the amount of which measures the force 

 exerted by the current. But this necessitates con- 

 stant manual control, and fails to meet one important 

 requirement in such instruments, — that they shall 

 give their indications both directly and continuously. 



Another mode of avoiding the use of a magnetic 

 needle, is illustrated by those instruments which em- 

 ploy the pulling action of a helical current upon a 

 rod of soft iron in its axis; and of these there are 

 very many forms. The volt-metre and ampere-metre 

 of Kohlrausch have the core in the form of a thin 

 tube of soft iron for liglitness, suspended by a rather 

 delicate spiral spring of many turns, similar to those 

 used in Jolly's specific-gravity balance. The iron 

 tube is closed at the top, and hangs over a cylinder 

 of non-magnetic material, which is fixed in the' axis 

 of the coil, and is of such a size as to leave a narrow 

 annular space between it and the iron. The air con- 

 fined within the tube thus acts like the liquid in a 

 dash-pot, but more freely, and damps the vibrations 

 of the tube and spring very effectually. An index 

 attached to the side of the iron tube moves in front 

 of a vertical scale, one side of which is graduated 

 experimentally to volts or amperes, the other in mil- 

 limetres. The zero-point is readily adjusted by means 

 of a sliding-rod and set-screw, from which the spiral 

 spring is hung. The two instruments differ merely 

 in the winding of the coils, the volt-metre having 

 many turns of fine wire, with a very high resistance; 

 while the coil of the ampere-metre has a iew turns 

 of very large wire, which has a resistance of but a 

 small fraction of an ohm. 



In the instruments of Ayrton and Perry, the same 

 mechanical action of the current is used; but the 

 indicator is novel, and very ingeniously uses the axial 

 twist, developed by longitudinal traction in a helix 

 formed from a ribbon of highly-elastic material. A 

 pointer attached to the end of the helix, or to a rod 



