Horn - should it have continuity? (Mark Rosenkrantz)

[rick hayllar]

HI all
Last time I pulled a horn apart was some time ago Last Millennium in fact
Consider though that they are powered by 12 Volt DC therefore something must 
happen to bring on the vibrations.
In fact  they operate just like an old fashioned door bell. There is an 
electromagnet  which when 12 volt is applied pulls the diaphragm (similar to 
a speaker diaphragm but made of steel) in one direction at the same time the 
moment of the diaphragm interrupts the circuit by opening some contacts 
causing the electromagnet to release the diagraph so it springs back to its 
rest position, thereby restoring the circuit and the whole process repeats 
itself a few hundred times a second. This vibration results in a very 
audible noise.
I remember actually tuning my horn a bit by adjusting the contacts usually 
by of a screw.
If you measure the impedance (resistance if the horn with an ohm-meter you 
would get a steady reading just what I don't know but I should imaging less 
than 100 ohms. If you apply a variable voltage to it and gradually increase 
the voltage than a some stage when then is enough power to operate the 
electromagnet than the horn will start to sound and a correctly operating 
will one give out a healthy sound at 12 Volts.
Hope this helps
Rick Hayllar