on the roof of JNR AC/DC EMUs

[miyakoji]

In this video, you can see three things on the roof above the driver's compartment on this 415 series very clearly when it comes to a stop at 0:23.  The smallest protrusion is a flare, the front-most is an antenna as far as I know, but the tallest of the three, I have no idea.  What is that?  It appears to have a ceramic insulator where it goes into the roof, and it seems only to be on some JNR-era AC/DC EMUs.  413, 415, 417 series have it, 717s don't.  I can't find it on any DC EMUs, nor AC-only EMUs which seem to be not that numerous, nor any locos (EF81s, which are AC/DC, do not seem to have it).  With that insulator, it's got to be electrical, but I can't imagine what it does.  Attract lighting?

 

[bikkuri bahn]

That is an electroscope antenna (kenden antenna or 検電アンテナ, in Japanese).  It detects AC current in an interchange section between AC and DC current.

[miyakoji]

Thanks Bikkuri.  I've seen some driver's station videos where the motorman is shown, at the break in electrical systems, putting the speed lever to 0 and then manually switching from one current type to the other.  Did these do that automatically?