Whats that sensor on the # 1 spark plug wire ??

[Andrew C]

I bought new spark plug wires for my `73 412 and I noticed that the new wires did not have a sensor on the #1 wire like the one I was replacing.I have gone and lost the old spark plug wire with the sensor on it and I was wondering what does that sensor do ? Is it just a radio spark suppressor ? If it is I won`t worry abot finding a new one . Thanks for any input ..Andrew C