246 



NATURE 



\yuly 29, 1875 



the galvanometer. Bend stout brass wire into flat-topped 

 arch and fix firmly in block ; the straight portion of wire 

 at top of arch having upon it a cork roller for raising or 

 lowering needles. Magnetise two sewing needles and fix 

 (with opposite poles adjoining) \ inch apart by means of 

 twisted fine copper wire. On same axis, \ inch above 

 upper needle, fix glass thread about 4 inches long to serve 

 as pointer. Suspend needles by silk fibre and attach 

 fibre to cork roller. Cut card into circle 4 inches diameter 

 and graduate circumference into degrees. Place (but do 

 not fix) card in proper position over coil, supporting it on 

 two corks cemented to board. Make needles as far as 

 possible astatic. Place them in position and cover all 

 with glass shade." 



After some preliminary work with the galvanometer, 

 a Daniell's cell and a simple form of Wheatstone's bridge 

 are made ; then a rheochord and a set of resistance coils. 

 Then comes the following work with these instruments, in 

 each case the necessary instructions being printed under 

 the work to be done : — 



" I. Measure relative resistances of different lengths of 

 the same copper wire by Wheatstone's Bridge. 2. Find 

 lengths of copper wires by measuring their relative resist- 

 ances, the length of one of the wires being known. 



3. Ascertain relation between resistance and weight. 



4. Ascertain effect of temperature on resistance. 5. 

 Experimentally establish the laws of divided circuits. 

 6. Measure the external resistance of your cell. 7. Com- 

 pare the electromotive force of your cell with that of a 

 Grove's cell." 



In this direction there is, of course, an almost unlimited 

 field for practical work, but other parts of the subject 

 claim attention, and the time that can be given to the 

 whole is extremely limited. Our space will not allow us 

 to detail further what is done in electricity, nor can we 

 give more than a hasty glance to the other subjects that 

 are taken up in successive years by the science teachers. 



Sound is not a very promising branch of Physics for 

 practical work; nevertheless, nine or ten days are use- 

 fully spent on this subject. A monochord is the piece de 

 resistance here, and when this is made the laws of the 

 transverse vibration of strings are verified, and the follow- 

 ing problems solved by its means : — " i. Weigh pieces of 

 metal of unknown weight. 2. The pitch of one tuning 

 fork being known, ascertain that of another unknown. 

 3. The diameter of a German silver wire being known, 

 ascertain its specific gravity." By means of the ordinary 

 shilling tuning forks some useful experiments are made, 

 and finally the velocities of sound in various solid, liquid, 

 and gaseous bodies are determined in different ways and 

 with a satisfactory approximation to the truth. This 

 will indicate merely the course of practical work in 

 sound. 



Heat and Light offer more facilities for practical work. 

 In Heat, a differential air thermometer is first made, then 

 an alcohol thermometer is determined and graduated ; the 

 maximum density of water is tried by simple hydro- 

 meters ; a bulb tube is made, and here we quote two 

 experiments in which this bulb is used for determining 

 coefficients of expansion :• — 



" Determine mean Coefficients of absolute expansion 

 of Water and Alcohol between temperature of the day 

 and 50° C. above. 



" Weigh bulb tube filled with liquid at temperatures 

 / and T. Calling weight of liquid at /, W and loss of 

 weight at J", w, the Coefficient of apparent expansion is 



~W _ 1JJ "^^^ '^^^l expansion is obtained by adding to 

 this the Coefficient of expansion of the glass. (See next 

 experiment.) 



" Determine mean Coefficient of expansion of glass of 

 thermometer tubing for 50° C. above the temperature of 

 the day. 



" Weigh bulb tube full of mercury at temperatures t 

 and Z", and so obtain Coefficient of apparent expansion 

 of mercury ( = E). Then assuming Coefficient of real 

 expansion of mercury as '00018 {^ C),C — B = mean 

 Coefficient of glass." 



The determination of specific and latent heat follows 

 this, and a few experiments on radiant heat conclude this 

 part. 



In Light a large range of subjects is available for 

 practical work, but the necessary instruments are more 

 numerous, and require rather more skill in their manu- 

 facture. Nevertheless several experiments will occur to 

 every teacher which can be made with very little pre- 

 paration, such, for example, as trying the law of inverse 

 squares, comparing in various ways the illuminating 

 powers of different sources of light, &c. Here is some- 

 thing rather more difficult : — 



" Make an instrument for measuring vertical heights by 

 reflection." Instructions for this are given, and the instru- 

 ment is then used for measuring the heights of ceilings, 

 doors, &c., after it has been fully explained. 



Silvering solutions are prepared and employed for 

 many purposes ; little concave and convex mirrors, for 

 example, are made out of large watch-glasses silvered by 

 this process of deposition, and the foci of these mirrors 

 are then determined. A movable model is made to 

 illustrate the law of sines ; and the index of refraction of 

 water is determined as follows : — 



" Graduate slip of glass about 8 CM. X i CM. to M.M. 

 Fix with sealing-wax two equal slips about 4 CM. long at 

 right angles to scale thus '— — 1 . Place in water so 

 that uprights are just below surface. Fix an eye-tube 

 (blackened inside) at some distance above water and in 

 line of scale, and note division at which top of nearest 

 upright appears on scale. Now carefully withdraw water 

 without disturbing apparatus, and again note division. 

 Let height of upright be //, and distances on scale from 

 upright respectively a and A, then - = tangent of angle 



of incidence, and |, = tangent of angle of refraction. 



From tangent calculate sines, using formula sine 6 — 



tan (9 



Index of refraction: 



Sine of angle of incidence. 



Sine of angle of refraction. 



Vi -f tan2 



Verify result by varying angles." 



A bisulphide of {carbon lens is made frorri two watch 

 glasses with ground edges, a notch being cut across to 

 introduce the liquid. A bisulphide of carbon prism is 

 not so easy to make ; here is the method recom- 

 mended : — 



" Cut-off and grind ends of glass tube about 2 inches 

 longX I inch diameter so that planes of ends make an 

 angle of about 60° with each other. Drill hole about ^ 

 inch diameter in middle of tube with hardened point of 

 triangular file and turpentine. Glue pieces of thin sheet 

 glass on ends. Fill with Bisulphide of Carbon and cover 

 hole with glued paper." 



By degrees a spectroscope is entirely built up, and with 



