102 



• KNOWLEDGE -> 



[Feb. 16, 1883. 



tho tail the polarization is more or less intense, according 

 to the position of tlie comet with respect to the earth and 

 sun. If the line joining the earth and comet makes a 

 small angle with the direction in wliich tlie sun's light 

 falls upon the comet, the polarization is small, Vnit the 

 intensity of the polarization increases as the angle between 

 the line joining the earth and comet and the direction of 

 illumination increases towards a right angle. This seems 

 to indicate that the polarization is duo to the dispersion 

 of the sun's light by particles whose diameters are small 

 compared with the wave-length of light. 



traversing one coil deflected the needle through 30 deg. to 

 the right, the same current passing through the other coil 

 would cause a deflection of .30 deg. to the left. If the cur- 

 rent is divided between the coils so as to pass through simul- 

 taneously or in " quantity," then there will be no move- 

 ment of the needle, as the forces exerted arc equal and 

 opposite. This may be more readily comprehended by a 

 reference to Fig. 1, in which Z C is the battery, the copper 

 pole being "put to earth," and the zinc joined by wire to 

 D, where the current divides betwen the coils A and B of 

 the galvanometer G. After leaving these coils, the current 



Great Comet of 1882, from a drawing by Dr. Temple, of Arcetri, IS Oct. 16 h. 



The phenomena exhibited by the recent brilliant comet 

 also affords evidence of matter streaming away from the 

 brighter parts of the tail towards the sun. Mr. Gill's pho- 

 tographs, as well as the drawings by Schmidt, Temple, and 

 others who observed in a clear atmosphere, show a broad 

 band of faint light which is traceable backwards from the 

 brighter parts of the tail far beyond the nucleus in the 

 direction of the sun. The band evidently does not spring 

 from the nucleus, as it is not symmetrically placed with 

 respect to it, but it corresponds with the broader and 

 brighter parts of the tail. 



According to my theory, the sun must be constantly 

 bombarded by matter driven ofi' from every meteoric mass 

 which comes into its neighbourhood, while a portion of the 

 less volatile elements are driven backwards into space. 



THE AMATEUR ELECTRICIAN. 



ELECTRICAL MEASUREMENT.— VIII. 



AS we intimated in the preceding article, it is some- 

 what difficult to measure the resistance of coils to 

 be subsequently used as measures of resistance in other 

 wires. Our position is in some degree akin to that of a 

 shopkeeper who has to make his own scales and weights, 

 all he has to guide him being a single one-ounce weight. 

 Given the ounce weight, he could make another similar 

 weight by restoring to the scale, by means of a sufficiently 

 large piece of metal, etc., the balance which had been 

 upset by the original weight being placed in one of the 

 pans. Further, he might possibly be able to produce his 

 second weight by causing the first to do a certain amount 

 of work, such as e.xtending a spiral spring, and then modi- 

 fying the dimensions of a piece of metal until it is able to 

 repeat that work, and so indicate an equality in weight. 

 The analogy in electrical measurement is very close. Instead 

 of a pair of scales, we use a " differential " galvanometer, 

 in which there are two coils of wire, each capable of exert- 

 ing an equal but opposite force upon a magnetic needle 

 suspended between them — that is to say, if a certain current 



* The plane of the comet's orbit was incUned at an angle of 

 about .38° to the ecliptic. The earth passed through tho plane of 

 the comet's orbit about the middle of September, so that at the 

 date of the drawing a straight tail with a Bcriea of carved tails 

 behind it in the pla-.io of tho orbit would bo seen in projection as 

 but slightly sepai-ated from one another. — A. C. K. 



passes direct to earth or back to the zinc pole of the 

 battery. If now we insert a known resistance (y) between 

 the coil A and the earth, it will be evident that the 

 " balance " will be upset, and that as the circuit A has a 

 higher resistance or lower conductivity than the circuit B, 

 there will be an unequal division of the current, the smaller 

 portion going through A, and exerting a comparatively 

 feebler effect upon the needle, which is then deflected by 

 the greater current circulating in B. In this case the de- 



flection is produced by the difference between A and B, 

 not by the whole current B, as the A current has still to 

 be neutralised. Suppose, now, we insert an unknown 

 resistance (.>■) in the B circuit between the galvanometer 

 and earth, and increase that resistance until the needle 

 again points to zero, it is clear that the two circuits again 

 offer equal resistances, and that the unknown x is equal to 

 the known >/. The differential galvanometer is a very 

 useful piece of apparatus, and will amply repay any trouble 

 expended in making it. The coils should be so nearly 

 equal as to be able to transmit a current from 40 or 50 

 Daniell cells without deflecting the needle. 



The other method available for the shopkeeper, we said, 

 was to make the new weight repeat work done by the 

 original. To apply this to electrical measurement, we wUl 

 refer to the tangent galvanometer described in " Electrical 

 Measurement — VI." (Knowledge, No. 58). If a current 

 from the standard cell (Knowledge, No. 44) be passed 

 through the quantity or thick-wire coil, a good deflection 

 will be obtained. The e.xternal resistance of such a circuit 

 is practically Jiil. If, therefore, we insert between the 

 battery and galvanometer a small resistance, it will ma- 

 terially reduce the strength of the current, and consequently 

 the deflection of the needle. By noting the deflection 

 resulting from tlie insertion of this known resistance, and 



