Hewittic Rectifier bulbs

I was able to see those English-made glass mercury arc rectifiers glowing through the vents in their case. The higher the current draw, the brighter they get and the louder the buzz. Being sealed in glass,with a rubber plug at the bottom, there is no air leak problem as long as they aren't broken. These run trolleys at Fort Edmonton Park in Alberta, Canada. pic taken 2006

Done