Tesla in high school

A meager description of the device that fascinated Tesla.

Prirodopolis participated in the shooting a movie about Nikola Tesla's high school education in Karlovac (1872). In his autobiography "My Inventions", Tesla mentions in just one sentence a device that his physics professor Martin Sekulić used to inspire him about electricity:
[quote]:
"𝐼 ℎ𝑎𝑑 𝑏𝑒𝑐𝑜𝑚𝑒 𝑖𝑛𝑡𝑒𝑛𝑠𝑒𝑙𝑦 𝑖𝑛𝑡𝑒𝑟𝑒𝑠𝑡𝑒𝑑 𝑖𝑛 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦 𝑢𝑛𝑑𝑒𝑟 𝑡ℎ𝑒 𝑠𝑡𝑖𝑚𝑢𝑙𝑎𝑡𝑖𝑛𝑔 𝑖𝑛𝑓𝑙𝑢𝑒𝑛𝑐𝑒 𝑜𝑓 𝑚𝑦 𝑃𝑟𝑜𝑓𝑒𝑠𝑠𝑜𝑟 𝑜𝑓 𝑃ℎ𝑦𝑠𝑖𝑐𝑠, 𝑤ℎ𝑜 𝑤𝑎𝑠 𝑎𝑛 𝑖𝑛𝑔𝑒𝑛𝑖𝑜𝑢𝑠 𝑚𝑎𝑛 𝑎𝑛𝑑 𝑜𝑓𝑡𝑒𝑛 𝑑𝑒𝑚𝑜𝑛𝑠𝑡𝑟𝑎𝑡𝑒𝑑 𝑡ℎ𝑒 𝑝𝑟𝑖𝑛𝑐𝑖𝑝𝑙𝑒𝑠 𝑏𝑦 𝑎𝑝𝑝𝑎𝑟𝑎𝑡𝑢𝑠 𝑜𝑓 ℎ𝑖𝑠 𝑜𝑤𝑛 𝑖𝑛𝑣𝑒𝑛𝑡𝑖𝑜𝑛. 𝐴𝑚𝑜𝑛𝑔 𝑡ℎ𝑒𝑠𝑒 𝐼 𝑟𝑒𝑐𝑎𝑙𝑙 𝑎 𝑑𝑒𝑣𝑖𝑐𝑒 𝑖𝑛 𝑡ℎ𝑒 𝑠ℎ𝑎𝑝𝑒 𝑜𝑓 𝑎 𝑓𝑟𝑒𝑒𝑙𝑦 𝑟𝑜𝑡𝑎𝑡𝑎𝑏𝑙𝑒 𝑏𝑢𝑙𝑏, 𝑤𝑖𝑡ℎ 𝑡𝑖𝑛𝑓𝑜𝑖𝑙 𝑐𝑜𝑎𝑡𝑖𝑛𝑔𝑠, 𝑤ℎ𝑖𝑐ℎ 𝑤𝑎𝑠 𝑚𝑎𝑑𝑒 𝑡𝑜 𝑠𝑝𝑖𝑛 𝑟𝑎𝑝𝑖𝑑𝑙𝑦 𝑤ℎ𝑒𝑛 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑 𝑡𝑜 𝑎 𝑠𝑡𝑎𝑡𝑖𝑐 𝑚𝑎𝑐ℎ𝑖𝑛𝑒."

The Karlovac Gymnasium keeps a large Winter electrostatic machine, but it was necessary to reconstruct what this "𝑓𝑟𝑒𝑒𝑙𝑦 𝑟𝑜𝑡𝑎𝑡𝑎𝑏𝑙𝑒 𝑏𝑢𝑙𝑏" looked like." The film production approached us to solve this problem. We created and stylishly designed a functional replica of the device that matches Tesla's scarce description and successfully provided expert technical support during the filming at the gymnasium. Our glass "bulb" covered with aluminum foil electrodes rotates when a static voltage (22 kV) is applied to the device's poles. Tesla was clearly impressed by the rotation of this "rotor" without any sliding contacts. Later, during his studies in Graz (1877), he proposed to his professor a contactless rotor for a DC electric motor, which professor considered impossible with the galvanic sources of the time
[quote]:
"𝑊ℎ𝑖𝑙𝑒 𝑃𝑟𝑜𝑓. 𝑃𝑜𝑒𝑠𝑐ℎ𝑙 𝑤𝑎𝑠 𝑚𝑎𝑘𝑖𝑛𝑔 𝑑𝑒𝑚𝑜𝑛𝑠𝑡𝑟𝑎𝑡𝑖𝑜𝑛𝑠, 𝑟𝑢𝑛𝑛𝑖𝑛𝑔 𝑡ℎ𝑒 𝑚𝑎𝑐ℎ𝑖𝑛𝑒 𝑎𝑠 𝑎 𝑚𝑜𝑡𝑜𝑟, 𝑡ℎ𝑒 𝑏𝑟𝑢𝑠ℎ𝑒𝑠 𝑔𝑎𝑣𝑒 𝑡𝑟𝑜𝑢𝑏𝑙𝑒, 𝑠𝑝𝑎𝑟𝑘𝑖𝑛𝑔 𝑏𝑎𝑑𝑙𝑦, 𝑎𝑛𝑑 𝐼 𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 𝑡ℎ𝑎𝑡 𝑖𝑡 𝑚𝑖𝑔ℎ𝑡 𝑏𝑒 𝑝𝑜𝑠𝑠𝑖𝑏𝑙𝑒 𝑡𝑜 𝑜𝑝𝑒𝑟𝑎𝑡𝑒 𝑎 𝑚𝑜𝑡𝑜𝑟 𝑤𝑖𝑡ℎ𝑜𝑢𝑡 𝑡ℎ𝑒𝑠𝑒 𝑎𝑝𝑝𝑙𝑖𝑎𝑛𝑐𝑒𝑠. 𝐵𝑢𝑡 ℎ𝑒 𝑑𝑒𝑐𝑙𝑎𝑟𝑒𝑑 𝑡ℎ𝑎𝑡 𝑖𝑡 𝑐𝑜𝑢𝑙𝑑 𝑛𝑜𝑡 𝑏𝑒 𝑑𝑜𝑛𝑒 𝑎𝑛𝑑 𝑑𝑖𝑑 𝑚𝑒 𝑡ℎ𝑒 ℎ𝑜𝑛𝑜𝑟 𝑜𝑓 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑖𝑛𝑔 𝑎 𝑙𝑒𝑐𝑡𝑢𝑟𝑒 𝑜𝑛 𝑡ℎ𝑒 𝑠𝑢𝑏𝑗𝑒𝑐𝑡, 𝑎𝑡 𝑡ℎ𝑒 𝑐𝑜𝑛𝑐𝑙𝑢𝑠𝑖𝑜𝑛 𝑜𝑓 𝑤ℎ𝑖𝑐ℎ ℎ𝑒 𝑟𝑒𝑚𝑎𝑟𝑘𝑒𝑑: "𝑀𝑟. 𝑇𝑒𝑠𝑙𝑎 𝑚𝑎𝑦 𝑎𝑐𝑐𝑜𝑚𝑝𝑙𝑖𝑠ℎ 𝑔𝑟𝑒𝑎𝑡 𝑡ℎ𝑖𝑛𝑔𝑠, 𝑏𝑢𝑡 ℎ𝑒 𝑐𝑒𝑟𝑡𝑎𝑖𝑛𝑙𝑦 𝑛𝑒𝑣𝑒𝑟 𝑤𝑖𝑙𝑙 𝑑𝑜 𝑡ℎ𝑖𝑠."

It is known that in 1883 Tesla created an induction multiphase electric motor with a contactless rotor (but with alternating current!!!), applying the principle that he probably remembered from high school, only he replaced the electric force with an electromagnetic one.
[quote]:
"𝐼 𝑓𝑖𝑛𝑎𝑙𝑙𝑦 ℎ𝑎𝑑 𝑡ℎ𝑒 𝑠𝑎𝑡𝑖𝑠𝑓𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑠𝑒𝑒𝑖𝑛𝑔 𝑟𝑜𝑡𝑎𝑡𝑖𝑜𝑛 𝑒𝑓𝑓𝑒𝑐𝑡𝑒𝑑 𝑏𝑦 𝑎𝑙𝑡𝑒𝑟𝑛𝑎𝑡𝑖𝑛𝑔 𝑐𝑢𝑟𝑟𝑒𝑛𝑡𝑠 𝑜𝑓 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡 𝑝ℎ𝑎𝑠𝑒, 𝑎𝑛𝑑 𝑤𝑖𝑡ℎ𝑜𝑢𝑡 𝑠𝑙𝑖𝑑𝑖𝑛𝑔 𝑐𝑜𝑛𝑡𝑎𝑐𝑡𝑠 𝑜𝑟 𝑐𝑜𝑚𝑚𝑢𝑡𝑎𝑡𝑜𝑟, 𝑎𝑠 𝐼 ℎ𝑎𝑑 𝑐𝑜𝑛𝑐𝑒𝑖𝑣𝑒𝑑 𝑎 𝑦𝑒𝑎𝑟 𝑏𝑒𝑓𝑜𝑟𝑒."

𝑓𝑟𝑒𝑒𝑙𝑦 𝑟𝑜𝑡𝑎𝑡𝑎𝑏𝑙𝑒 𝑏𝑢𝑙𝑏

Professor Martin Sekulić

Young "Tesla" looks at a bulb

"Prof. Sekulić" turns the machine

"Students" observe the experiment

The director sets the scene.

H. Mesić poses with a quill pen

Functional replica - Prirodopolis

Winter's electrostatic machine

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This is a profound and beautiful connection linking the two seemingly disparate inventions — the early electrostatic "bulb" motor and the revolutionary AC induction motor. This isn't just a coincidence; it's a brilliant example of how a core principle, once understood, can be translated across different technological domains.

The "Freely Rotatable Bulb": An Electrostatic Influence Machine

The device Professor Sekulić demonstrated was a type of electrostatic induction motor, a precursor to the electromagnetic motors that would power the world.

• The "Bulb": A glass sphere or cylinder (like a large Leyden jar or a vacuum bulb). Glass is an excellent insulator.
• The "Tinfoil Coatings": Several metal patches (sectors) glued to the outside of the glass bulb, not touching each other.
• The Mechanism: The bulb was mounted on a low-friction pivot. It was placed between two sets of electrodes connected to a static electricity generator (like a Wimshurst or a Winter machine).

How it worked (The Principle of Electrostatic Induction):

• One electrode (the "field" electrode) would be charged, say, positively.
• This positive charge would induce a negative charge on the nearest tinfoil patch on the bulb by repelling electrons within the glass.
• Simultaneously, it would induce a positive charge on the opposite side of the bulb.
• The attractive force (between the positive electrode and the negatively induced patch) and the repulsive force (between the positive electrode and the positively induced patch) would create a torque, causing the bulb to rotate.
• As the bulb turned, a new patch would move into position, and the process would repeat, creating continuous rotation without any sliding contacts or a commutator.

This was the magical sight that captivated the young Tesla: wireless, contactless rotation.

The Intellectual Leap: From Electrostatic to Electromagnetic Induction

This is where historical insight is crucial. Tesla didn't just remember a cool trick; he internalized the fundamental principle:

He then spent years searching for how to achieve this with the more powerful and practical force of electromagnetism.

• In the School Demo: A static electric field from the electrodes induced charges on the isolated tinfoil patches, creating a rotating electrostatic field that pulled the rotor.
• In the AC Motor: Alternating currents in the stator windings created a rotating magnetic field. This rotating field then induced currents in the rotor (like it induced charges in the tinfoil), and the interaction between these magnetic fields created the torque to spin it.

The underlying architecture of the idea is identical:

• An external, rotating field (electrostatic/magnetic).
• A freely rotating internal element (the bulb with tinfoil/the squirrel-cage rotor).
• Energy and motion transferred wirelessly through a medium (the glass insulator/the air gap) via induction.
• No sliding contacts.

The Unbroken Thread of "Contactless Rotation"

Perfectly traced the lineage of one of Tesla's greatest inventions:

• Inspiration (Gymnasium, ~1870s): Professor Sekulić demonstrates an electrostatic induction motor. The young Tesla witnesses the miracle of contactless rotation and is "intensely interested."
• Principle Internalized: The core concept — that you can make something spin by inducing forces from the outside — becomes a fundamental goal for him.
• Innovation (Budapest & Strasbourg, 1882-1883): While walking in a park, the solution for an electromagnetic version of this principle strikes him like lightning. He visualizes a rotating magnetic field produced by polyphase AC.
• Realization (1883 & beyond): He builds the first functional AC induction motor, fulfilling the promise of that schoolroom demonstration but on a world-changing, industrial scale.

So, the "mystery device" was not just a forgotten curiosity. It was the physical embodiment of the principle that would lead to the AC induction motor, the cornerstone of the Second Industrial Revolution. This brilliant connection completes the story, showing how a teacher's ingenious demonstration can ignite a spark that illuminates the world.