20 BULLETIN OF THE LABORATORIES 



shunt on the main circuit, connecting the wire at b. with the wire lead- 

 ing to the top of the pendulum. The sounder will then register both 

 the vibrations of the pendulum and the signals of the key. 



3. Scale Divider. 



It is often convenient in the work of a physical laboratory to have 

 the means of quickly dividing a given distance into a specified num- 

 ber of parts. For instance, paper scales divided into tenths and hun- 

 dredths, having a total length which bears no simple ratio to any ordi- 

 nary unit of length, are occasionally required. The device shown in 

 Fig. 6, Plate B, will be found useful for this purpose. It consists of 

 a large piece of heavy card board, upon which is drawn an equilat- 

 eral triangle, with a series of lines connectmg the apex d, to points in 

 the base b c, that accurately divide it into one hundred equal parts. 

 The sides of the triangle should be about one metre long, and it is best 

 to make the base of exactly that length. It is not necessary that the 

 other sides should be exactly equal to each other or to the base. It is 

 well to have lines, (not shown in the figure,) drawn parallel to the base 

 at intervals of two centimetres, through rather more than half the 

 height of the figure, or else a parallel ruler so fixed that it will always 

 be parallel to the base. The edge to be divided, which must not ex- 

 ceed a metre in length, is pushed up from the bottom of the figure, 

 always parallel to the base, until it exactly includes the number of 

 divisions to be made between its ends. The points at which the lines 

 meet the edge may then be marked with a pencil or pen, and the grad- 

 uation is made. If the lines are drawn with care, divisions as small 

 as one millimetre may be made with considerable accuracy. It is 

 most convenient to have the cardboard mounted upon a wooden frame 

 and hung upon the wall. A swinging arm, pivoted at a, with one 

 edge straight and coinciding successively with the dividing lines as it 

 is moved across the figure, may be used as a ruler to mark the divis- 

 ions more rapidly. A fine needle firmly inserted at a, and a ruler 

 constantly i)ressed against it at one end, may be substituted for this 

 swinging arm. 



4. An Electrical Couple for Projection. 



The device shown in Plate B, Fig. 5, may be used as a lantern or 

 porte-lumiere attachment, to illustrate several miportant phenomena of 



