Best way to improve Ruhmkorff coil performance and prevent battery overheating?

I have been trying to build a ruhmkorff coil. How do I improve it? Should I connect a resistor between primary coil and battery? Sometimes I think battery gets warm. I think I attached a drawing of the schematic as a file. I am not trying to build a flyback converter or flyback transformer, I am trying to build a ruhmkorff coil. The device consist of a secondary coil, primary coil, 2 interconnected lithium ion cells, serve as the battery and a multimeter to take current readings.

Can't see your schematic. Traditionally these were a self oscillating device, since a pulsed or square wave current drive on the primary is required, and an interrupter circuit (switch) on the primary side was driven by the magnetic field from the iron core. The ignition coil in a car uses the contacts in the distributor to switch the primary current in older cars, in modern cars the switch is solid state. A resistor is not desirable, since it limits the primary current, however, if the primary current is greater than the capacity of the battery, the battery voltage will drop and the battery will get quite warm. A DVM will have problems measuring the current, since it is pulsed. Did you design and build the coil, or is it a car coil for example.
Cheers,
Richard

Ruhmkorff patented his first coil design in 1851, but after viewing an improved version by the American Edward Ritchie made refinements for his own second ...

An induction coil or "spark coil" (archaically known as an inductorium or Ruhmkorff coil[1] after Heinrich Rühmkorff) is a type of electrical transformer[2][3][4] used to produce high-voltage pulses from a low-voltage direct current (DC) supply.[1][5] To create the flux changes necessary to induce voltage in the secondary coil, the direct current in the primary coil is repeatedly interrupted by a vibrating mechanical contact called an interrupter.[1] Invented in 1836 by Nicholas Callan, with additional research by Charles Grafton Page and others,[1] the induction coil was the first type of transformer. It was widely used in x-ray machines,[1][6] spark-gap radio transmitters,[1][6] arc lighting and quack medical electrotherapy devices from the 1880s to the 1920s. Today its only common use is as the ignition coils in internal combustion engines and in physics education to demonstrate induction.

The primary behaves as an inductor, storing energy in the associated magnetic field. When the primary current is suddenly interrupted, the magnetic field rapidly collapses. This causes a high voltage pulse to be developed across the secondary terminals through electromagnetic induction.