The Alternate Scientist

Building A Jacob's Ladder


Mike burst in through the door of my basement workshop and yelled, "Aha, caught you in the act! Don't move. I have you surrounded. What are you doing with those coat hangers?"

"I'm cutting the hooks off and straigtening them out," I replied calmly.

"Rats! You destroyed the evidence. Well, I'll get you next time." He flopped onto the old leather couch against the side wall. "What're you doing that for anyhow?"

"If you must know, I'm building a Jacob's ladder."

"Who is Jacob and why does he need a ladder?" asked mike quizzically.

"Jacob is a second story man and he's going to pull a job tonight," I replied without hesitation.

"Now I understand," said Mike leaning back on the couch and startling a mouse that jumped from underneath and scurried across the floor. "Whoops! Sorry Herman Mouster."

I had finished straightening the two coat hangers and proceeded to bend them into identical shapes. Then I slid a heavy object to the center of the workbench.

"If that's your lunchbox, it's a little rusty," commented Mike.

"This," said I, "is a neon sign transformer. A bit rusty I'll admit."

"I can paint it for you," Mike offered.

"No thank you." I had seen Mike's paint jobs. He would sit it in his driveway and, without even cleaning off the rust, paint the whole thing; including I.D. plate, insulators and terminals. "It works fine the way it is. It puts out 12,000 volts at 30 milliamperes - that's thirty one-thousanths of an amp."

"12,000 volts!" he sat up straight. "I think I'll just sit on the ground on the other side of this wall. A long way on the other side."

"Just stay put! If used with reasonable caution this transformer is not dangerous. Anyway, the TV you like to watch so much has over twice that much voltage on the picture tube."

He relaxed again. "Oh, well, what's a measly 12,000 volts? But why are you putting those coat hangers on it?"

"These are my electrodes." I said in my best Karloff voice. "They are for shocking my monster."

"I can get better shocks at the service station for $12.95," stated Mike flatly.

"All right smart aleck. How long of a spark can you get from 12,000 volts?" Mike shrugged. I continued, "The breakdown voltage of air at sea level is 22 volts per mil; that works out to 22,000 volts per inch."

Mike raised his hand. "A little over half an inch?" he asked hopefully.

"Exactly right," I said and tossed him a pistachio from the bowl on my computer desk. He deftly caught it in his mouth, cracked the shell free with his teeth, spat the shell into his hand and chewed the nut. "So I am going to form these electrodes so that they are just under half an inch apart at their closest."

"How exactly do you form them?" asked Mike.

"That's just a twenty-five cent word for bending." I said and preceded to do so. I soon had the coat hangers in a long, narrow 'V'; about three inches wide at the top and a half inch at the bottom. I pulled a suicide cord from a nail on the wall, connected the alligator clips to the primary terminals of the transformer and plugged the other end into the switched outlet on the side of the workbench. I reached for the switch.

"Am I safe here or should I remember that I left my bathtub running?" said Mike uncertainly. I ignored him and hit the switch.

With a sharp crack, a blue spark shot between the coat hangers at the bottom of the 'V'. It hesitated half a second then began to rise. In another second it had climbed three quarters of the way up the 'V', gotten longer and whiter until it vanished with a soft pop. That was instantly followed by another crack and the cycle repeated itself.

I turned to Mike triumphantly but the couch was empty! Then a head slowly rose from behind it. "What's wrong with it?" he asked. "Why is it sparking like that?"

That's exactly what it's supposed to do," I replied.

"Why?"

"Do you remember that I made the gap at the bottom of the 'V' a little smaller than the maximum this voltage could jump? So when we apply power to the transformer an arc is immediately struck at the shortest gap. The air, heated by the arc itself starts to rise and carries the arc with it.

"But won't it extinquish when it gets longer than about half an inch?" Curiosity was getting the better of Mike.

"Since the current has already establihed an ionized plasma, that plasma can slowly be lengthend, to a point, without breaking."

"But eventually it gets too long and breaks?" Mike was coming toward the bench, his eyes fixed on the humming, cracking spark.

"That's it. And then a new arc is struck at the bottom and the process repeats.

"But why doesn't go all the way to the top?"

"My guess at the maximum spacing was a little generous. Let me bend the closer together." I shut off the switch on the side of the bench then pulled the A.C. plug from the transformer and layed it in plain sight on top of the bench before I bent the coat hangers slightly.

"Belt and suspenders, huh?" observed Mike.

"That's right. 12,000 volts even at only 30 mils is extremely dangerous! I have other experiments planned and I don't want this one to be my last." I plugged the A.C. cord in, took a step back form the bench and flipped the switch. The Jacob's ladder performed as before except that the arc rose all the way to the top and arched upward substantially before it broke.

"That's pretty slick," observed Mike. "Is that spark as hot as it looks?"

I tore a page from the tablet I keep handy on the workbench and held a corner of it in the moving arc. In a few seconds the paper burst into flame. "Convinced?" I said to Mike, dropping the paper on the concrete floor and stepping on it.

It's hot all right! What do you use a Jabob's ladder for?"

I picked up the paper and felt the burned corner with my fingers to make sure it was cold before dropping it into the wastebasket. Mostly in old Frankenstein movies," I replied. "But it makes a spectacular demonstraion as you can see and illustrates a few basic priciples."

"Like the current will take the shortest path, hot air rises and the rising air can drag the arc with it into a longer and longer path," stated Mike.

"There is one more thing I observed by accident that concerns whistlers..."

"You're going to have to save it," Mike said looking at his watch. "If I'm not home in time for supper, my mother is going be up in arms." With that he ran out the door, leaving it open as usual, and jumped on his bicycle.


Tom Hunter (N3CRK), 24 NOV 2000