1905 - 6 .] A Neiv Form of Harmonic Synthetiser. 215 
passes along some 18 inches to the receiving drum. The latter is 
caused to turn by means of a weight which is sufficiently heavy 
to make it wind up the paper and keep it taut as it passes from 
the one drum to the other, but is too small to interfere at all with 
the action of the winding drum, which alone controls the rate of 
motion. The object of having a space of 18 inches between the 
pen and the receiving drum is simply to allow a considerable 
number of the waves traced out to be seen simultaneously by the 
operator. This is always convenient, but it is of course specially 
so when any progressive change in the wave-form is being studied, 
or when it is desired to observe the effect produced by gradually 
altering the period or amplitude of one of the constituent 
harmonics. 
§ 5. Details of Construction. 
It may be well to mention certain points of detail. The pulleys 
employed are made of brass, they are carefully turned up, and 
have a diameter of about an inch. The axle of each pulley is 
pointed and runs between the hollow ends of two screws, an 
arrangement which allows good adjustment to be easily made, 
without shake on the one hand or stiffness on the other, and 
which also has the useful effect of minimising friction. 
There are no slides anywhere throughout the machine : each 
moving part turns on pivots, even the rectilinearly moving pen, 
and no doubt to this fact is due the smooth and successful work- 
ing of the apparatus ; which has thus justified the hope that, if 
properly designed, a simply made instrument of the kind would 
be quite satisfactory. 
It has been found that the use of a leather belt of circular 
section, engaging in the V-shaped grooves of wooden wheels, which 
are nowhere less than 4 inches in diameter, has obviated any 
trouble that might arise from slipping. The plan adopted of 
having only one main driving belt, with a simple arrangement to 
keep the tension constant, gives no trouble, and is a great simpli- 
fication. The small German-made motor employed to drive the 
apparatus is sent out by the makers furnished with a pulley 
which is driven from the armature shaft by a worm gear, and 
the use of this pulley at one stroke gives the necessary reduction 
to a slow speed. 
