Figure 1: Skeletal formula of sodium potassium tartrate

Now, we know Rochelle salts form a lattice, which is a repeated, ordered symmetrical framework of the units in the diagram above, but it must also be mentioned that the units of these structures are hydrated; specifically, it is a tetrahydrate [3]. That is why it was essential to mix the water with the tartrate, as that is what builds water molecules into the crystal. The function of the water molecules is to help space out the ions evenly and stabilize the lattice [4]; this is essential for the piezoelectric properties of the Rochelle salt. 

With our newly made crystals, we must test them for piezoelectricity. In order to do this it is important to understand what the piezoelectric effect is, and how the chemical composition of rochelle salts allow this effect to exist.

Rochelle salts and Piezoelectricity

Rochelle salts have been studied for their piezoelectric effects for a long time. Piezoelectricity simply means being able to generate electricity through applied mechanical stress. When mechanical stress is applied to piezoelectric crystals, a voltage is generated [6], as shown in Figure 2. 

Figure 2: Drawing depicting the effect of applying mechanical stress on a Rochelle salt (left) lattice as opposed to the Rochelle salt in its resting state (right)

Piezoelectric materials cannot have a centre of symmetry [7]. In Figure 3, on the left, is the non-piezoelectric unit, identifiable by its centre of symmetry. On the non-piezoelectric side, the central atom sits right in the middle, and the surrounding atoms are evenly spaced. As a result, the whole structure is centrosymmetric (mirrored, balanced), this causes any charges to cancel out and the symmetrical structure puts it at a state where it experiences minimal electrostatic repulsions [6].

On the piezoelectric side (right), the central atom has shifted slightly off-centre. Now one side of the unit has a little more positive charge and the opposite side has a little more negative charge. This imbalance creates a polarisation, or a separation of charge within the unit. The uneven charge is what creates the voltage [8]. 

This means in piezoelectric crystals, the unit cell is slightly asymmetrical, so what you’ll be seeing throughout a lattice of a Rochelle salt would be the unit cell on the right. The positive and negative charge centres do not line up exactly, meaning each unit cell has a tiny electric dipole (a small separation of positive and negative charge). When stress is applied to a Rochelle salt, the atoms physically shift relative to each other. The lattice is asymmetrical, so the shift increases or decreases the separation between the oppositely charged centers, and as a result these tiny electric dipoles change. This is happening across billions of dipoles and if we imagine them all aligning, the whole crystal’s surface develops a net polarisation (buildup of positive charges on one face and negative on opposite face), creating an electric potential across the crystal [6].

Thus, if mechanical force is applied to this Rochelle salt by squeezing it, and the faces of a Rochelle salt are connected with a wire, this will create a potential difference where one side is positive, and the other side is negative. If you connect a wire either side of the Rochelle salt (one end is connected to the side that’s positive, and the other end connected to the negative side), then the electrons will flow through the wire to balance the charge difference. 

The Walkie Talkies

As we mentioned before, the Rochelle salt was wrapped in a coil of copper wire. During their distant communication, Senku had to carry around a metallic bulky box with antennae while speaking to Kohaku through this Rochelle salt. The metallic box almost certainly hides a stone-age oscillator and modulator circuit. An oscillator circuit is an electronic circuit that produces a stable repeating waveform without needing an input signal [9], and a modulator, alters a carrier signal from an oscillator using audio or data [10]. The oscillator (the object making the consistent sound) in Senku’s box makes a high-frequency AC carrier: a rapidly oscillating current that switches between the positive and negative terminals of a wire. Essentially, imagine electrons jiggling back and forth across a wire instead of actually moving across it [9]. If Senku spoke into his Rochelle salt microphone without the oscillator and modulator, the Rochelle salt microphone on its own would emit frequencies too low for efficient radiation. A high frequency wave can radiate well from antennas, so that is why a high frequency AC carrier is used. The modulator “imprints” Senku’s audio signal onto that carrier. Therefore, after Senku has spoken into his Rochelle salt microphone, his voice creates a voltage on the crystal that matches his waveform, the sound waves compress and stretch the Rochelle salt crystal, inducing the piezoelectric effect, and creating a voltage (caused by net polarization) that matches his voice waveform. This voltage drives a current in the coil of wire, and the current in the coil oscillates in sync with his voice. The oscillating current basically creates radio waves. The coil and attached antenna radiate these radio waves into the air, allowing them to travel a long distance and fly through space. Kohaku’s earpiece has a Rochelle salt wrapped in a coil of wire. Together this setup forms a crude receiver circuit that accepts Senku’s sound waves. As a result, the salt crystal earpiece acts as a speaker through a reverse piezoelectric effect. On Kohaku’s end, it induces a voltage and makes the crystal expand and contract in sync with the signal, the tiny vibrations shake the thin plate pressed against her ear, and this creates sound waves, Kohaku as a result can hear Senku’s exact words.