ELECTROMAGNETIC UNIT OF ELECTRICITY TO THE ELECTROSTATIC UNIT. 607 
We will now describe the different pieces fixed to the axle beginning from the left 
of fig. 4. 
SCALE or CENTIMETRES. 
First there is a wooden disc with two grooves for the driving string, next comes the 
wooden disc on which the stroboscopic pattern is painted. The end view of this is 
shown in fiof. 5. 
Beyond this are the two commntators, which are exactly alike, each is made of two 
portions of brass tube fixed to an ebonite bush. Two grooves are turned in the con¬ 
tinuous portion of the tul)e, in which rest two wires by means of which electrical 
connection is made with the parts of the commutator. Although the slits in the com¬ 
mutator were not more than about 2 mm. wide yet no trouble at all was experienced 
through want of insulation. The commutator could be easily cleaned by scraping the 
ebonite between the parts of the commutator with a sharp tool. There was also very 
good insulation between the commutator and the axle. The insulation was tested 
several times by means of the gold leaf electroscope, and was found practically perfect. 
Following the two commutators is an ebonite disc to prevent oil ifojn the centies 
getting to the commutators. 
The last thing on the axle is an endless screw in which gears a wmrm-wheel of 
30 teeth. This wheel is furnished with a ])in which makes contact with a spring 
once in every revolution of the wheel, i.e.. every 30 revolutions of the commutator. 
The spring is insulated from the framewmrk, and the contact of the pin with it 
completes the circuit required for exciting one of the electromagnets of the recording 
apparatus. The spring js not shown in the figure. 
