[Illustration: FIG. 1.-This shows the principle of this wonderful Galvanometer invented by Lord Kelvin in its latest form. Current enters at _a_, passes round the coils, as shown by the arrows, and away at _b_. A light rod, _c_, is suspended by the fine fibre, _d_, so that the eight little magnets hang in the centres of the coils--four in each. The current deflects these magnets and so turns the mirror, _m_, at the bottom of the rod. At _e_ are two large magnets which give the little ones the necessary tendency to keep at "zero."] [Illustration: FIG. 2.--Here we see the working parts of the "String Galvanometer," by which the beating of the heart can be registered electrically. The current flows down the fine silvered fibre, between the poles, _a_ and _b_, of a powerful magnet. As the current varies, the fibre bends more or less.] The main body of the instrument is a large, powerful electro-magnet, in shape like a large pair of jaws nearly shut. Energised by a strong current, this magnet produces an exceedingly strong magnetic field in the small space between the "teeth" as it were. In this space there is stretched a fine thread of quartz which is almost perfectly elastic. It is a non-conductor, however, so it is covered with a fine coating of silver. Silver wire is sometimes used, but no way has yet been found of drawing any metallic wire so thin as the quartz fibre, which is sometimes as thin as two thousandths of a millimetre, or about a twelve-thousandth of an inch. A hundred pages of this book make up a thickness of about an inch, so that one leaf is about a fiftieth of an inch. Consequently the fibre in question could be multiplied 240 times before it became as stout as the paper on which these words are printed. The current to be measured, then, is passed through the stretched fibre and the interaction of the magnetic field by which the fibre is then surrounded, with the magnetic field in which it is immersed, causes it to be deflected to one side. Of course the deflection is exceedingly small in amount, and as it is undesirable to hamper its movements by the weight of a mirror, no matter how small, some other means of reading the instrument had to be devised. This is a microscope which is fixed to one of the jaws, through a fine hole in which the movements of the fibre can be viewed. Or what is often better still, a picture of the wire can be projected through the microscope on to a screen or on to a moving photographic plate or strip of photographic paper. In the latter case a permanent record is made of the changes in the flowing current.