THE GALVANOMETER. 



227 



through him from his feet to his head ; let him turn his face to the 

 needle : its north pole is always deflected to his left side." 



The ** Tangent" Galvanometer consists of a vertical circle like a:i 

 upright ring, across which is a support in the centre holding a copper wire, 

 through which the electric current passes. On this point (where the wire is) 

 a needle is very lightly supported, and when the instrument is to be used it 

 is placed so that the plane of the circle is parallel to the line in which the 

 needle points. The current passes, and the needle is deviated. By noting 

 which side the north end of the needle goes the direction of the current is 

 ascertained, and the length of the needle being small in comparison with the 

 diameter of the circle through which the current passes, the strength of the 

 current in the vertical circle is in proportion to the tangent of the angle 

 through which the needle turns. Hence the term "Tangent" Galvanometer. 



The " Reflecting " instrument is the invention of Sir William Thomson, 

 in which a mirror is attached to the needle, and reflects a small focus of 

 light upon a scale. The movements, however minute, are easily read. Sir 

 W. Thomson's Galvanometers are extremely sensitive. We need not men- 

 tion any other varieties, as full descriptions can easily be obtained. We 

 only need to indicate the mode of working. 



The accompanying illustration (fig. 231) shows an Astatic Galvanometer 

 which may be used in two ways either to 

 measure strength of current, or to find out a 

 current ; in the latter case it would be termed 

 a Galvanoscope. It is a compound needle 

 instrument, and consists of two needles placed 

 in parallel directions with opposite poles 

 above each other in a coil. The wire coil 

 is wound round a bobbin, and the astatic 

 needle is placed therein and suspended 

 freely, as in the illustration, by a cocoon 

 thread. The upper needle moves upon a 

 scale, O O, and the instrument is enclosed 

 in a glass shade. The screw, V, communi- 

 cates with the upper needle, and fixes it at 

 zero point when ready for use. The wires 

 are fastened to the binding- screws, and the 

 current is then sent. The needle is deflected Fig - 23I> -Galvanometer, 



accordingly, and the number of degrees on the scale can be read off. 



The uses of the galvanic current are many. Amongst them Electro- 

 plating is perhaps the most generally useful, though Electrotyping is also a 

 very important process in art. A visitor to Birmingham may view the pro- 

 cess carried on there by some enterprising firms, who have succeeded 

 wonderfully in producing electro-plate. The principle is very simple and 

 easy to understand, but the greatest care and watchfulness are required on 

 ihe part of the men employed. The principle, as we have said, is simple, 



