Oct. 31, 1878] 



NATURE 



707 



The additions to the Zoological Society's Gardens during the 

 past week include two Macaque Monkeys [Matacus cytiomolgits) 

 from India, presented respectively by Capt. E. Waterhouse and 

 Mr. Samuel Thomson ; a Common Roe (,Capreolus caprea) from 

 Greece, presented by Mr, Edward Jones ; a Common Jackal 

 {Cants auretts) from India, presented by Capt. Easson; a 

 Common Seal {Phoca vitulina), European, presented by Messrs. 

 Thompson and Gough; a Bornean Fireback {Eiiplocamtis 

 ttohUis) from Borneo, presented by Mr, A. Dent ; two Mandarin 

 Ducks {Aix galericulata) from China, presented by Mr. Edward 

 Trelawny ; a Common Marmoset {Hapala jacchus), a Tuber- 

 culated Lizard {Iguana tuber ctilata), a Teguexin Lizard {Teius 

 t€guexin), a Merrem's Snake (Liophis merremi), a Black-headed 

 Snake {Homalocranion melanocephalum), a Plumbeous Snake 

 {Oxyrhoptis plumbeus), a d'Orbigny's Snake (ffeterdon d^orbignyi), 

 an Anaconda [Eunectes murinus) from South America, pur- 

 chased ; a Collared Fruit 'Ba.t{Cynonycieris collaris), bornGin the 

 Gardens. 



A NEW GALVANOMETER FOR STRONG 

 CURRENTS 



/~\N the following principle an ordinary tangent galvanometer 

 ^-^ can be transformed into an instrument suitable for the 

 measurement of strong currents such as produced by powerful 

 magneto- or dynamo-electric machines. 



Suppose the circular coil of a tangent galvanometer mounted 

 so as to turn round its horizontal diameter lying in the meridian, 

 and assume the needle to be freely movable in all directions, 

 then the effect which the current produces upon the magnet at 

 different inclinations of the coil to the horizontal plane is as 

 follows : — 



1st, If the ring is in the vertical position (in the meridian) 

 we have the ordinary form of tangent galvanometer, for which 



k I 

 tan a = -jj (l) 



where o is the deflection of the needle in the horizontal plane, / 

 the strength of the current, k a constant depending upon the 

 dimensions of the coil, and ZTthe horizontal component of the 

 earth's magnetism, 



2nd. If the ring is in the vertical position the magnet is only 

 deflected in the plane of the meridian, and the deflection is 

 determined by 



kl ^'^ 

 tanp = -p- (2) 



where /3 is the deflection and V the vertical component of the 

 earth's magnetism. This would be a tangent galvanometer in 

 which the directive force of the current is opposed by the 

 vertical component of the terrestrial magnetism. 

 By the combination of these two formulte we obtain 

 tan g _ V 

 tan fi~ I?' 

 Hence, the tangents of the two deflections are in inverse pro- 

 portion respectively to the two components of the earth's 

 magnetism. 

 V 

 Since ^= tan i, where / is the "magnetic dip," this relation 



may be used to ascertain the *' dip " by a method similar to that 

 of Prof. Wilhelm Weber by the inductive action of the earth. 



3rd. If the ring is neither in the vertical nor in the horizontal 

 position, but is inclined at any angle <p to the horizontal plane, 

 the magnet is simultaneously deflected from the plane of the 

 meridian through an angle a and from the horizontal plane 

 through an angle ^. In this case we have to introduce, instead 

 of k in the equations I and 2 respectively, k sin <p and k cos <p, 

 whereby they become 



kl 



H 



kl 



V 



tan a = 



tan)8 — 



sm<^ 



cos (^ 



(3) 

 (4) 



Combining these two equations we obtain the formula 



tan 

 taniS 



cot ^ = _ = tan i, 

 H 



<p being knowa and a and j3 read off, the "dip" may be found 



by such measurements without altering the inclination of 

 the coil. 



If the ring is gradually brought from the vertical to the hori- 

 zontal position, whilst a current / passes through it, the deflec- 

 tion a decreases proportionally from the maximum tan a = — - 

 to zero. At the same time the deflection j8 increases from zero 

 to the maxunum tan fi = - 



For practical measurements we need only consider the deflec- 

 tion a in the horizontal plane, and for this reason the needle 

 should work on a vertical axle pivoted at both ends. 



With this form of instrument I was enabled to measure very 

 strong currents. 



It will be readily understood that a current which would 

 throw the needle to nearly 90° when the ring is vertical, will, 

 when it is suitably inclined, only deflect the needle to that part 

 of the scale (45°) where readings are most accurate. 



If the instrument and place of observation remain the same, 

 we can substitute in equation (3) a new constant IC for 



— whereby it is simplified to 



tana = JT/sin <p. 

 Further we have for other currents Z^, Zj, &c., at other 

 angles of inclination ^j, (p2, &c. 



tan Ui = A'/^ sin (py 

 tan 02 = -H^/^ ^^'■^ 'Pn ^'^•f 

 ... = 7 sin <|> : /j sin ^1 : I2 sin (j>2, &.C., 

 T . T . T — ^^^ "■ . ^^'^ "i . ^'^^ "2 



" ^ ' 2 ' * * sin (^ sin ^-^ ' sin </>2 " ' 

 By this relation different currents measured at different incli- 

 nations of the ring can be compared. 



The following separate cases may serve as further illustra- 

 tions : — 



Case I. Currents of different strength /j, 1^ /j . . ., sent 

 through the coil at the same inclination ^, give — 



tan « : tan a^ : tan o^ . . . — I : I^\ I.^ . . . 

 Therefore the law of tangents holds also for the inclined ring. 

 Case 2. The same cm^rent / sent through the ring at different 

 angles of inclination ^, (p-^, <p.2 • . • gives 

 tan a : tan a^ : tan a^ . . . = sin <J) 

 tan a.. 



hence, 



tan a : tan o^ : tan o^ 



tan a 



tan Oj 

 sin (pi 



sin 4>2 



sm (pi : sin ^2 

 . . . = C 



sm <t> 

 where C a constant. 



The tangents of the deflections are therefore in the same pro- 

 portion as the sines of the inclinations ; or in other words, the 

 tangents of the deflections divided by the sines of the corre- 

 sponding inclinations give for the same strength 'of current a 

 constant value. 



Case 2. For different currents I, I^, I„ . . . sent through the 

 ring at inclinations <p, <py, tp.^ . . . giving the same deflection a 

 (say of 45°) we have 



j.r . 7 . _ I . I . I 



-^ • ■*! • -*2 • • • • : • —; • ' • * • 



sm (p sm <Pi sin (p^ 

 = cosec <p : cosec <pi : cosec <p.2 ... 

 and the instrument thus used acts as a cosecant galvanometer. 



The instrument which I used to ascertain the degree of 

 accuracy of the method described consisted of a wooden ring of 

 30 cm. diameter, wound for some experiments with a few convo- 

 lutions of wire, and for other experiments with a copper band. 

 This ring, in the centre of which a small magnetic needle was 

 placed, could be turned round its horizontal diameter, and its 

 inclination read off on a graduated quadrant. To adjust the 

 instrument the ring is approximately placed in the horizontal 

 position ; a current is then sent through the coil, and if the 

 needle is deflected from the meridian, the inclination of the ring 

 must be carefully altered until no deflection occurs. In this 

 position the quadrant is fixed so that its zero point coincides 

 with the index attached to the coil, and the instrument is now 

 ready for use. 



The following tables contain records of some of the experi- 

 ments made with this instrument : — 



Table I. gives the results obtained with a coil of seven convo- 

 lutions of wire of "074 Siemens' units resistance, and with a 

 needle turning on a point. One Bunsen's cell was used, and the 

 strength of current varied by the introduction of resistances. 

 For each ciu-rent-strength readings were taken at inclinations of 

 the ring, the sines of which are proportional to the even integers 2 

 to 10. 



